I feel like you don't really GET how this works

A regional pokedex isn't just "new mons of the region"

The Sinnoh Dex's problem in Diamond & Pearl lied more in its weird type distribution, most infamously with the Chimchar and Ponyta lines being the only fire types actually present.

It did have one upside though in that gyms became based more around move types than Pokémon types, which IMO offered a better challenge since you could no longer just sweep through every gym with a few moves that were super-effective against the given type, and had to think on your toes to adjust for the different Pokémon types spread across the gym leaders' and Elite Four's teams.

Still, the fact that regional type distribution was as bad as it was in Diamond & Pearl is pretty laughable.