If I'm understanding this correctly the competition wouldn't be dev vs dev but store vs store. If that is the case then your app becomes a commodity and each store will try to compete on price, lowering the dev's profit margin in the process.
In other words, nobody will be forcing these developers to be on a particular store -- unlike Apple's App Store, where you have to play on their terms.
If someone could launch a competitor to the Apple App store, and say "I'm going to take half the margin that apple takes from app developers", I think developers would flock to that store, and then so would customers.