The problem goes deeper than this though.
Say we do what you're suggesting. There's now three stores on Android devices when you take them out of the box: Google Play, Crumbt, and Dipple. Crumbt quickly gains a reputation for hosting sketchy software, with some diamonds in the rough, but users also regularly report getting malware on their phones. That said everything is free, which makes it the #1 stop for people who have to buy the down-market phones anyway because they can't afford apps either. Dipple, meanwhile, promises no AI features or content, and immediately gets a following with the more monied crowd that not only will pay for apps, but prefers to to avoid ads and spammy generative content.
The problem, as I see it, is we still have the old, bad experience from google, and we now have one other experience that lets anything in, so people are still getting malware, and we have still a third that's offering premium features to users with cash to spend and leaves everyone else out in the cold.
I know I'm being negative with my examples, but at the same time, the PROBLEM, as I see it, is power asymmetry: lack of oversight on the part of the platforms means nasty shit gets through, and lack of actionable routes to be remediated means the risk lands squarely on the user, who is disempowered already. Additional app markets solve none of that.
Like, sure, an app market could come along promising to be on the user's side: it will immediately not attract developers, because developers will (correctly) see that it's riskier there than elsewhere. And there's not really a monetary reason to make an ethical app store, whatever that happens to mean, because far more money is made being an unethical one that hangs it's users out to dry.
I just don't think these are problems yet more markets can solve, because fundamentally, what's broken about them is what makes them so wildly profitable. And I can't fathom another store not coming along and, even if not at first, doing the exact same thing again anyways that Play has already done, because it works. Because Play itself was immitating/innovating on the same axis as Apple's App Store.