Cena was over in OVW as "Prototype", Batista was over as a sea monster. So should they have carried these gimmicks up to the main roster as well, since you think that superstars and Divas should have no gimmick change and be exactly the same character that they were in the development leagues?
OVW had nowhere near the popularity or exposure that NXT does.
When you're showing something is successful on a national and international level why change it when calling them up to the main roster?
OVW and FCW had nowhere near the reach that NXT has. When wrestlers were coming up from those places WWE wasn't mentioning that they were in those places.
Two very different animals there.
People like Kevin Owens and Paige came to the main roster and didn't need changes and they're doing fine. Sasha Banks didn't need to be changed and she's doing fine. Tyler Breeze kept his same gimmick and is going to be in the mid card where he belongs.
I've already mentioned that The Shield was a great change for those guys.
But someone like Charlotte who was already one of the most over women's wrestlers in NXT didn't need the change.
The Ascension were very over in NXT with the gimmick they had. They didn't need to be made into a cartoon like generic Road Warriors wannabe team.
The Neville comic book thing was fine because it didn't deviate from what made him popular in NXT which was his in ring stuff. So the comic book stuff was a nice subtle addition to what he was already doing. If they had gone full Hurricane on him though it would have been pretty dumb.
And obviously NXT has stupid stuff that should be changed like when Emma came to the main roster and was doing her idiotic dancing. But I'm sure Vince loved that and thought it was great.