Being able to trade your old mons into the new games just makes it way too easy to keep reusing the same team in every new game. This actually forces you to rethink your strategies, because your favorite mons might not be available. It also makes you appreciate the new mons more.
I mean, this info is literally just coming out, so there's a good chance they just want people to focus on Gen 8, its mechanics, etc, until 2020,
Essentially, yes, that is the plan. By September, we'll have a better idea....and possibly LE Switch for Gen 8 that will sell out faster than ChuChu and Eevs Lesssss Gooo LE's.
Can't say if that pokeball accessory will make it over....you know, that's a lot of programming