Okay, joke aside, let's be honest with each other and say that even when Sakura got the Byakugou seal people were complaining that it didn't make sense. Exactly as you are saying now, people were saying about that too.
There wasn't a demonstrable prerequisite which Sakura couldn't prove as having prior to developing her seal like there is for SM. Tsunade's explanation of the requirements for medical ninjutsu included considerable academic knowledge, advanced control your stamina, and the willingness to never give up. It stands to reason that with the Yin Seal being the pinnacle of medical ninjutsu that Sakura could maybe achieve it by persisently refining her chakra control to perfection and learning what is necessary to develop the fuinjutsu.
Sakura gets a boost in skills and even though we are given an explanation, people will still complain that it is rushed or doesn't make sense.
To be fair, there was never anything to prove or indicate that Sakura would develop the Yin Seal besides her status as apprentice. The notion of young surpassing the old could have been fulfilled through her unique methods. In fact, that's what many of us would have preferred but Kishi apparently disagrees.
More often than not people give Sakura a hard time with this, but Sasuke has done it several times and not really explained why or how he got such a power, but people never complained about it that much. We just assumed "plot device" and moved on.
On the contrary, many of his power ups are explainable. The chakra tree walking we were given a narrative portrayal of his progress as he refined his chakra. Later he trains for a month to develop the Chidori and increase his agility which Kakashi analysed using his Sharingan. The latter I'll give you as being somewhat abrupt with how long it took Lee to develop that grade of speed but speed is an indefinite physical stat with the only requirement being conventional training of your body. That's more easily defined than Sakura being the bodily superior of Orochimaru or implementing external chakra from a seal to use SM when the process has been shown to take place whilst chakra is being moulded.
His P2 jutsu as Orosuke doesn't actually need an explanation, does it? He had 2 1/2 years to learn kenjutsu (something which Orochimaru was well versed in since Kusanagi was a major part of his fighting technique), expand his Raiton arsenal (being the ninjutsu enthusiast/scientist that he is with years of research under his belt, it's definitely possible that Orochimaru could have trained Sasuke to develop the Raiton jutsu that he did), perfect the usage of the Curse Seal (Orochimaru is the progenitor of CS), and train his Sharingan to the best of its capacity (this is something which Sasuke should be capable of doing on his own).
If your grievances against Sasuke's method for attaining power have do with MS then you should curse the system of Mangekyou Sharingan in general and not Sasuke. I'll give you that the hawk summoning was convoluted though. Even Kishi seemed to be aware of how random it was when he drew Tobi as curious about how and when he could have made a contract with hawks.
For me, Sakura gaining the byakugou seal is not that surprising considering she is Tsunade's student and knowledge of techniques is a passed down trait from one generation to the next. I would have been more surprised of she didn't get the seal and remained at the power level she was.
It's not too surprising for me either actually. Naruto inherited the Rasengan from Jiraiya and Sasuke learned Orochimaru's kenjutsu and he signed a contract with the snakes. The major grievance against that is the execution or a lack of foreshadowing.
As for the Sage mode, did we ever find out how Kabuto got the Snake sage mode? Not really. All he did was fuse himself with Orochimaru's DNA and boom he got the mode.
I don't see how Orochimaru's DNA would enable Kabuto to gain SM or how the loss of that genetic material would prevent him from continuing to use it. Something that's overlooked by many people is the fact that Kabuto injected himself with the DNA of Taka and the Sound 5. That includes the original source of the Curse Seal which is capable of passively gathering natural energy and Karin's Uzumaki vitality. Yes, losing Orochimaru's abilities would drastically weaken Kabuto but I don't see how he wouldn't be able to continue using SM, albeit a weaker state of it without the abundance of snakes or Orochimaru's jutsu at his disposal (snake summonings should still be available like Manda 2.0 since Kabuto has a contract with them but he'll need to perform summoning jutsu like Naruto). Unless Orochimaru gathering his chakra somehow drained Kabuto of the knowledge to use senjutsu or if the chakra of the other subordinates was taken despite there being no mention of all the DNA he had implanted being removed, just Orochimaru's.
I'm digressing here though. The point is that Kabuto's situation has a logical basis to itself. He has the DNA of two (possibly three or more) people that would enable him to gather natural energy and manage it efficiently.
No training shown, no idea where he got it from, it was just assumed he got it from Orochimaru.
Orochimaru was confirmed to be dead at the time and Kabuto had disappeared from the manga for over one hundred chapters. There was a huge window there for him to do some off-screen training at Ryuchidou. That's not inherently illogical because he had the tools, the knowledge, and the time to train in senjutsu.
As for why Orochimaru didn't use it is a mystery.
This was explained in #579. Orochimaru ventured to Ryuchi Cave in the hopes of developing Sage Mode but he failed because he lacked an efficent enough body. That's after having spent years, possibly decades modifying his body to make it sturdier and more long lasting. You'll forgive me if I find it convoluted that a girl touted as being physically and historically "normal" as more capable than someone with far greater chakra reserves and bodily integrity.
It is quite possible that when he did the body switch technique the body he inhabited didn't have enough chakra reserve and so he couldn't perform it.
I don't believe the personal reserves of the body he's inhabiting is relevant. It's just a shell for him, Orochimaru's actual form is the white snake. He can make use of the body's form and techniques since Orochimaru supposedly inherits their knowledge as well, but the person who is Orochimaru is capable of existing independently of the vessel. Someone with inadequate chakra reserves would definitely not be capable of the liberal usage of taxing jutsu like the snake based replacement technique.
I could even theorized that he may have it on all the time which would explain his deformities and at times why he has to switch bodies all the time to prevent himself from being assimilated. That would explain a lot actually like how he is able to turn himself into a snake.
You do understand that all of this was explained during Orochimaru's so-called "fight" with Sasuke in the Itachi Pursit Arc, right? The snake body and deformities are a result of the modifications he made to himself and transferring his essence to various bodies. The origins of his mutanthood were defined hundreds of chapters ago.
As for how can Sakura achieve such a thing...well if Kishimoto wanted, so it shall be.
So now we're just going in circles with the logic of Kishi pointlessly allowing it to happen just because without given any rational thought for why it was possible in the first place.
We have to keep this reality open and understand that this can happen.
A writer should be allowed to take some creative liberties with their work and suspension of disbelief is a given but in a world as intricately written as Naruto's that has an established system and method of logic for itself, one which Kishi has consistently abided by for the great majority of the manga's life, the creator should follow what they have established with the utmost consistent. Moreover, there have been no inconsistencies in the method of acquirement for SM or the qualities of those who managed to develop senjutsu. Why must that fact be unnecessarily tarnished just to give a power up to someone who doesn't even need it?
Hell, if Kubo can make Ichigo part Quincy just by saying "His mother was a Quincy" then why not? There are worse asspulls in the field of manga out there that make even little sense.
I don't know who those characters are but just because the creator of that universe is willing to discard stipulations or laws that exist in his work doesn't mean that Naruto should emulate it.
About Sakura getting SSM;
It's not a debate if it makes sense or not. It's on whether it's likely or not(logic makes it likely, but think of Kishi's writing style). In Kishi's case, two different things.
It's interesting to see how complacent some people are with discarding already established narrative concepts when it benefits the characters that they personally like. But even in the matter of probability, I would argue that it's not likely given that Sakura just received chakra comparable to Sage Mode along with the rest of the Alliance.
Of course it should've been even more foreshadowed, and I have no idea when she would have time to train for it. Does this decrease the possibility? ..Not enough to make me surprised if she gets it.
Let's not forget that she doesn't even meet the physical requirements for SM. I must admit, Sakura getting SM would be very abrupt.
