I understand what you mean, but considering this is Kishi we're talking about, I wouldn't put it past him.
-The sannin fight. Why hasn't Jiraiya used the sage mode?
-The chuunin exams. Why hasn't Naruto used the kuchiyose no jutsu? It would've been an easy win.
-The Two Hokages vs Sarutobi. Why hasn't Hashirama used the sage mode?
I think what these fights have in common is that these characters have an even bigger battle coming up. Therefore they don't get the merit of showcasing all their abilities when they should. I could probably think of more but I'm tired now...bleh....
- With Jiraiya I think it was because Tsunade had poisoned him beforehand so he was having problems with chakra.
- With Naruto, he still had some problems with the summonings, he had only previously summoned Gamabunta thanks to the kyuubi chakra.
- And I think Orochimaru purposely restrained the Hokages back then so that they wouldn't go out of control. This time around Hashirama had enough chakra to casually defy the control of the edo-tensei.
I get what you mean about not showcasing abilities but this is different since it's not a new ability. It's the same Sakura punch, just with more power.
The punch she did in this chapter was like 10 times greater than the previous ones. By that logic Sakura would've been fighting at only 10% capacity this whole time. That doesn't make any sense to me, so I can't accept that's what she did. But that's just me.