>>58480562RPGs need character casts to make the world feel expansive and lived in. If there is a small amount of characters, it would feel a little empty. Gym Leaders were never meant to be characters that are important to the plot, they add to the world and gameplay by challenging you with specific challenges for you to comprehend the mechanics. Wanting every single character to be developed is more of an expectation people now have thanks to what Unova's games had to offer. It's why an XY sequel ended up having almost no XY characters
A good Pokemon game should at least have the rival friend group, champion and main antagonist fleshed out so we have a good story to chew on with these players. Since this game has no villains, I don't know if they really succeeded with most of the "antagonist" factions (Team Rust, Jett, Floette, fucking idk)