It doesn't take many to saturate those markets though.

If you're looking for a good indie shooter, look at Diabotical[0]. It's more Quake than Quake Champions was, or even Rocket Arena for that matter. There's plenty of pro-level gameplay on Zoot's stream[1] as well.

0: https://www.diabotical.com/

1: https://www.twitch.tv/thisiszoot

Maybe not "indie," but you don't have to look to the big 4 publishers for the best FPS and open-world action games these days.

Witcher 3 and Cyberpunk 2077 came out of CDProject Red. My favorite FPS of the last few years (Hunt: Showdown) came out of Crytek.

Yeah, open world games generally take a large team to create tons of content and a large tools/pipeline team to get said content in engine. So yeah, I doubt we'll see them dominating that space any time soon.

I can't think of a good reason though that there aren't a few very successful indie shooters though.

Mount & Blade series is an interesting counterexample to that, although I'm at a loss as to how define the genre ("feudalism sandbox"?). The first games were very clearly indy, but they capitalized on that success, and Bannerlord is a much more ambitious and polished game.

Open world as in sandbox? Sandbox is a very big genre among indie games.