Earlier quoted context omitted.
I could see Microsoft saying "we're only allowing apps installed through our 'store', for safety/security reasons, unless you opt out (gated by some scary warning that doing so is unsafe). Even if they never charged a fee for running the store, I bet this would raise a lot of eyebrows.
Microsoft has been going in the opposite direction. Nowadays you can post any win32 app to the store, they are loosening not tightening
My point was more about how people would react if MS did such a thing (i.e. installs to come from the store by default).