You don't think there is a material difference between Microsoft changing Windows to a closed platform and Apple creating a closed platform from scratch?
In the case of the latter it is well known and can be planned around. In the case of the former you have fully invested and then suddenly the game changes.
It won't happen though. The minute that Apple closed off the Mac, with the Mac App Store being the only software source, the minute that idea might have a little credence.
The Mac App Store is not (yet) the only software source, though that restriction is available as an opt-in setting in System Preferences.
Applications from outside the App Store run just fine, as long as they're signed with a developer certificate. The certificate presumably costs $99/year, but you don't need Apple to approve your application.
Furthermore, you don't technically need the certificate. The deafest are protective but the system will happily launch unsigned apps if you right click them.
In the case of the latter it is well known and can be planned around. In the case of the former you have fully invested and then suddenly the game changes.