Because it’s hard to support employees and compete with corporate behemoths like Microsoft and Apple when your product is a free, open-source OS?
Personally I haven’t heard of anyone getting support from Microsoft or finding Microsoft help pages useful. MacOS and Windows are making money on the support for corporate users and for manufacturers preinstalling the system (Apple being it for themselfs). Nothing that Linux cannot also do.
We are talking about going mainstream, then do you think that if Linux would have ~80% of the desktop market, there won’t be any commercial support companies and normie level help? There certanly is for the server space, even home servers like NAS devices.
I don’t get the point of playing what if.
If Linux somehow grew its market share to 80% of all users then there probably would be some form of support-based business or companies forking off their own version and building their own supported platforms, and the we end up with a bunch of closed platforms competing for all the money by offering a more polished experience for a premium.
Or none of that happens. I don’t know, this is all just make-believe because it’s a scenario that’s never going to happen.