So long as keeping 70% of 100 sales is better than keeping 100% of 50 sales, I just can't see competing app stores getting big.
I don't assume it would fix all problems. I just assume it would force Apple to move to a slightly more developer/consumer friendly model.
Also, in a lot of cases, the choice is 100% of 0 sales because Apple just won't let you in their store.
For an example of what a third party app store would look like, check out Cydia app store (the main way to get apps on your jailbroken phone). There are a ton of useful apps in there.
For example, if every app that didn't like the 30% cut rule went to another app store that didn't have that rule, that other app store would either get really popular or Apple would be forced to change their fee structure.