Doesn't that just clear out the low-quality, low-effort competition for the better scammers? And create a stronger presumption that any given person is not a scammer, because otherwise Apple/Microsoft/Google/Amazon/whomever would have kicked them out?

People might forget that "caveat emptor" still applies, even in a walled garden.

The huge number of fraudulent and malware-ish apps for android vs iOS does suggest that costs reduce the number of low quality attackers. I guess that is good for protecting the naive user. But I'm more concerned about protecting against the threat that will take your whole digital identity.