Nope, it's more like you are not understanding what I'm saying and just desperately trying to stick to your own agenda.
Where did those negative energy vines come from. Oh yeah Zygarde, when Lysandre was manipulating it with its own energy beam. Those vines caused a lot of destruction in Kalos. When Zygarde transformed into Perfect Zygarde, after it destroyed Lysandre's Megalith, it released an energy aura which reversed all the damage done. But despite that, some negative energy vines remained. Zygarde asked for Greninja's help to destroy those remaining negative energy vines, since Greninja has the power to sense the vines.
How can those negative vines come back once Zygarde and Greninja destroyed all those vines? That's nonsense. Once a negative vine is destroyed, it's done for, and doesn't anymore. Are you trying to say that the negative vines have unlimited source of supply? LMAO. Zygarde clearly asked for Greninja's help to deal with the remainder of the negative vines, so it's obvious that at one point, the vines will all be destroyed.
Pidgeot role is never done at all, since it's role is to protect the Pidgey-Pidgeotto. Even if the Pidgeottos evolve into Pidgeot there's absolutely no guarantee that the flock will stay protected, them could be under attack from .a stronger mon. Pidgeot's role is to protect the flock, so it always has to stay with the flock and give its services to protect the flock. Greninja's job wasn't stated anything like protecting Kalos, it's job just was to deal with those negative energy vines, Zygarde asked for Greninja's help in that matter because: Greninja has the power to sense those negative vines while Zygarde doesn't, that's it. It's Zygarde's duty to protect Kalos overall, not Greninja's.
Seriously stop being so desperate to fit your agenda that released Pokemon can't come back for Ash. That's point is outright proven wrong when Goodra, despite being released in XY070, still came back in the Kalos League when Ash needed it for a full team.
Do I even need to explain what a regional ace is? The Pokemon among Ash's regionally caught mons(Pokemon caught in a specific region) which turns out to be most powerful among those regional Pokemon and gets the most limelight/spotlight in that particular region compared to others. Look at Charizard, Sceptile, Infernape for example.