The problem is that most normal people (HN is not normal - mostly for the better) don't even understand what sideloading is - let alone actually care.
How can we fix this?
(aside from making people care - apathy enables so many political problems in the current age, but it's such a huge problem that this definitely isn't going to be the impetus to fix it)
The irony of that iconic Apple 1984 add .
microsoft wishes they could have the level of platform control that google/apple on mobiles have.
It's pure luck that the IBM-compatible PC was not locked down and restricted, because at the time IBM had not thought of it as being important. When it became clear that it was a lost profit opportunity, the cat was already out of the bag and so IBM had no choice.
Microsoft repeated the same "mistake". But apple learnt, and google also from apple.
Nowadays Microsoft could easily do it, they aren't fully into it, because they managed to botch themselves the whole WinRT/UWP and Windows 10X transition, had they made it in a way that most Windows developers would join the party, and the outcome would look much different.
Windows 11 sandboxing already requires MSIX and store distribution to be fully enabled, they only have to slowly keep turning the knobs on Windows 12 in whatever form it shows up, eventually.