Well, OP confused several concepts.
I admit that gimmicks are different from basic system design. But this is a process which is still developing constantly. Early gens(from 1 to 4) mainly designed the whole battle system and made it complete, which was more like giving a skeleton. The latter gens added muscle and blood(the so-called gimmick) to generate a lively character(the balanced and stable battle systems now). And GF knew that the system cannot just accomplish the addition calculations, and they needed time to also solve the subtraction problems, or the complex and redundant system would explode in the end.
Gimmicks make a lot of sense. In Gen6, you need to consider every mega when you form your team, and the battle is mega-focused. In Gen7, Z-moves bring more balance to the mega environment. While now, pokemon are more inclined to learn steel or ground type moves due to the Dynamax gimmick. All of the above are battle influences derived from the new gimmicks. I'm looking forward to other new gimmicks in the future gens.
They've basically been evolving and iterating rather than revolutionising each generation. And that's fine. It has progressed, it just hasn't massively shaken up
I will argue that Megas, Z-Moves and Dynamax HAVE progressed battles. It's a shame Megas & Z-Moves aren't presently in Gen 8. However, Megas were in SMUSUM and were heavily used in battles. They didn't just drop it then, despite many seemingly asking them to. I do hope Z-Move and Megas do return in the main games. We know they're going nowhere: Mega Evolution is to be shown again in the anime 2 weeks after it comes back and both exist in one form or another in Masters (Z Moves are essentially Sync Moves in Masters).
Megas and Z -Moves changed things up, you had to think about it in multiplayer. Who should I have as a Mega? When should I drop the Z-Move? Dynamax opens this up even further by having any Pokémon capable of doing it without the need for a held item. It makes more Pokémon usable. Hell, I'd even argue the best Z-Moves aren't the ones that do damage, but ones that boost up your Pokémon.
Hell, look at Durant. Who'd use Durant really? However, it is seeing increased use because of Dynamax.
Adding more and more systems will end up with coupling problems, and debugging will become more difficult. Also, focusing on one new gimmick should not be influenced by older sub-systems, because this will limit the design process for the new features.
