I can absolutely see them doing this... and SOON. Give the MAS ecosystem a year, two at most, to mature.
There WILL be a switch somewhere, in the system prefs... a defaults write command... why? Because of Adobe, ProTools, Microsoft, and to a lesser extent stuff like homebrew and macports.
If there is a switch then there’s no problem. There would be no social battle. (Some people would be annoyed but that’s about it.)
I can absolutely see Apple adding such a switch and activating it by default. It wouldn’t be a big deal. (Well, I think Apple technically already added that switch. As far as I know Parental Controls can be set up in a way to only allow apps from the App Store.)
There WILL be a switch somewhere, in the system prefs... a defaults write command... why? Because of Adobe, ProTools, Microsoft, and to a lesser extent stuff like homebrew and macports.