No, i have no such assumptions. Developers can and some may even will (though market forces will push at least one app store to be the dominant - chances are it'll be the one that comes out of the box with the device) focus on a single app store and there is nothing wrong with that.

The only wrong part is users not being in control over what they can install on their devices and from where they will install them.

Yes, it is possible that people will be forced to install applications they do not want, but this isn't something that is or even should be up to the OS vendor to try and protect against - the person being forced to do that is the one who should and if they cannot, then legal action should be seeked. The entire issue is outside the scope of the OS.