I would imagine that they can enjoy the nostalgic games of their childhood, relive those memories, and then not have the Pokemon they used be stuck there?
I'm not sure what you're not getting, here.
New pokemon, new world, new story, the works. It's a new Gen. You can split hairs and call it Gen 6.5 depending upon the circumstances if you want to, but it's a new Gen all the same.
So pseudo nostalgia. Great.