That's hardly a stable solution. It would start out nicely, but some of them would start eating up the market due to network effects. Then turn evil...

If you make network effects harder (e.g. it extremely easy to submit apps to all of them), I don't see this not becoming a competition over price. And security is expensive. You'd have to prevent people search some nice apps on your store, then downloading the same on a different store with lots of shit tier apps as well, if it saves them your 30% tax.