Toad, one of the most annoying characters of all time.
Now imagine him saying your princess is in another castle twenty times.
Are we going to lose Lucario?
Are we going to lose Pokémon Trainer?
Are we going to get Zoroark or his generation 6 equivalent?
Are we going to get Mewtwo back because of his new form?
All have to be considered.
Newtwo bridges the gap between between generation 6 and generation 1.
However that would make 6 gen 1 Pokémon if Pokémon Trainer stays. But is a gen 6 transformation good enough to represent the series?
Pikachu is staying because that is Pokémon's icon.
Jigglypuff is a Super Smash Bros. staple now. I think she is staying.