Except everyone pokemon game defines end game as after Elite Four and so do almost all of the Pokemon community.
And what's wrong with gyms? This isn't even really adding story it's adding gyms to challenge while in the Sevi Islands. Would definitely help e4 and end game training of pokemon. They could be made completely optional as well, as they aren't monumental or a necessity, just like going to Kanto/Hoenn is optional.
The area is pretty void of content or things to do already, except breed, get cash, or get Johto pokemon, all things available in other areas where there is more to do. Adding this would give more purpose and a reason othere than extra exp and cash, breeding, and Johto pokemon to do.