Let's be fair - I don't think we're talking about simply swapping lists when you zoom out. At a minimum, any value prop would have to match the existing major app stores such as verifying binary sources, rejecting malicious apps, and the like.

I think the main question I see is - do multiple stores benefit the user?

I'm not sure of that answer but I think we can agree that multiple stores do NOT help the default app store, which in turn could be beneficial to the consumer (multiple stores that have to compete on pricing w/ deals, self publishers offering a cheaper price directly, etc. - think more like grocery stores selling the same stuff vs farmers market vs direct from farm).

I'm no economist but I think we could also agree that having at least a few options is generally A Good Thing.

edit: regardless, even in a world with multiple stores the point re: attack surface is a good one and one of your other comments regarding what users actually value like safety is an important one, which as a business are the things you need to weigh on to make a profit