I don't think the issue is with them having stayed with the same style for so long, but rather they haven't really made it feel rewarding putting effort into delving into the mechanics as they are now. As
Tapu Koko Loko said, Pokemon's turn based battle system relies on knowing their Pokemon's specs (Stats, Ability, Moves, Type Match-ups, Held Item), damage calculation, predicting what your opponent has or is going to do, and calculating risk vs reward. It is a really deep system with many generations worth of Moves, Abilities, and Items to flesh it out and provide potentially limitless options...
BUT the games NEVER require you to do this. Okay, maybe the Battle Facilities will require you to have a winning strategy, but unless you're versing another human opponent the AI is usually simple and doesn't use any strategy except the type of Pokemon they're using. And no, I'm not asking to have Battle Facility level trainers during the main game, but I'm asking to see them at least using strategies that makes me think more carefully about how I set up my Pokemon. But since they don't there's no reason for me to dive into the deep battling mechanics as surface level stuff is good enough to get me by and going deeper doesn't change anything.
You know, the reason we haven't seen a Battle Frontier since Gen IV is because GF says the majority of players weren't playing it so they felt it wasn't worth doing. But I'm wondering if this is because, since the Battle Facilities expect the players to start using strategy, it's not that players don't like it but weren't prepared because the main game didn't require them to be so feel like this is a "difficult spike". Which in that case the solution wasn't removing the Battle Frontier but to add strategies to the trainers in the main game.
The reason this indepth battling system feels so bland is entirely GF's fault for not wanting to put the effort to make players use more complex strategies during main game. Thus players aren't prepared for Battle Facilities thus players don't want to play it thus GF feeling their efforts were wasted thus GF makes the game simpler... BREAK THE CYCLE GF! The issue is on your end!