Careful? Where's the fun in that?
The only OS Pokemon other than Pikachu that had any real personality was Charizard... it literally took the Orange Islands and Johto region to give Bulbasaur and Squirtle actual character. Not that I expect Ash's OG bird to ever make a comeback, but it not having a personality is hardly a fault against it.Of course not, Pidgeot's never coming back. And I don't think more than two people want it to, Pidgeot didn't have any personality unlike all his other OS Pokemon. That's why nobody cared it left.
Your fair point is fair, but it's still released the exact same way Pidgeot was... to protect something and no trainer around in sight. There is a way to make it believable if it ever comes back, although how believable really comes down to exactly how the game treats Zygarde.Greninja however will.
1. It's a starter
2. Popular as heck
Just cause it probably isn't happening this saga doesn't mean it won't.