1.) A legit mainline Pokemon game was already confirmed at E3.
2.) What if there are two generations of Pokemon on Switch, like there were on DS and 3DS?
1) Legit, in my book, either means a new type of triple A budget open world game, or an HD new version of the handheld games. Pokemon Stadium or any other thing they may call "mainline" wouldn't count, but I get your point and i am inclined to agree that their definition of mainline is the same as ours.
2) Then they will canabolize each other.