Well story wise the only reason Nagato couldn't revert back to sharingan was so that it would be revealed later the truth about it when we also found out who obito was.
That's a valid reason to explain the reasoning for not reverting it for Kishi as a writer but not as an in-universe explanation.
QUOTE
But it's also possible that Nagato never even suspected it could revert back to the sharingan. All of his friends and master told him that those eyes were of their own making special and no one suggested they be linked to the sharingan. But if Obito knows the truth then he could use his rinnegan eye as second sharingan and have access to the other doujutsu.
If Tobi could access the Sharingan then he would he not have done that instead of risking injury like when Kakashi had discovered the specifics of his teleportation? Instead of endangering himself by utilizing a jutsu which Kakashi is capable of counteracting why not use Susanoo or the other MS technique that that eye should possess?
QUOTE
Anyway the way things are going Madara and Obito are persons of habits. Even thou they have the rinnegan they use sharingan abilities. Maybe it's also a form of making them different from one another.
From we have been provided so far it would appear that for whatever reason Tobi and Nagato are incapable of deactivating the doujutsu that they were implanted with. It can probably be attributed to the same cause for Kakashi and Danzo's inability to revert their Sharingan eyes back to base form. Anything else is mere speculation that doesn't really assist in explaining these other circumstances concerning Kakashi, Danzo, and Ao who also possess implanted eyes.
QUOTE
No, i'm saying is that rinnegan and sharingan are different doujutsus
They have different appearances and different abilities but biologically the Rinnegan is related to the Sharingan. It is the final evolutionary step of the doujutsu. What analogy can I use here... It would be like a Poliwhirl evolving into Politoed. Though it does appear and it may have different mannerisms with the means to acquiring it being uniquely separated from its previous forms it's still biologically related to those creatures.
QUOTE
and i said that based on this even a Senju can unlokc the rinnegan if with only the Uchiha's DNA,
I believe I already addressed this in my previous post. The two clans received different inheritances, the Uchiha being provided with the sage's eyes. The Senju were not provided with those eyes neither do they possess the biological proclivity to develop the Rinnegan without them. Think of it as you would a puzzle. The Senju are one half of it whilst the Uchiha's eyes are the second. To complete the puzzle (Rinnegan) you need the other's doujutsu, their inheritance which is the second half. Otherwise you are unable to complete it. DNA alone is insufficient because it does not provide the eyes which the Senju require if they want to obtain Rinnegan.
QUOTE
and it proved that Rinnegan is not an evolution of Sharingan
Really? Then how do you explain Madara's EMS evolving into Rinnegan when he became decrepit? Or the various times that he transformed his eyes from Rinnegan to EMS and vice verse? See the pokemon analogy that I utilized above to answer this.
QUOTE
it's just an another doujutsu that both Uchiha's and Senju can unlock it infusing their DNA on each other to complete the puzzle and become like the sage.
In terms of abilities and appearance it is separate but biologically they share considerable familiarities since it is their eyes mixed with Senju DNA that enables the transformation in the first place.