So, if a "main series" Pokémon game is coming 6 months after LGPE is released, why should I care about this Go hybrid?

I mean, that's basically a remake/spinoff.
The fact that you can trade with future games doesn't really matter since even Go will be able to do that.

To me this looks like an introduction game for normies that only play Go and don't even know what Serebii or Bulbapedia is.