That would not be possible because Macs are much more open by design. There are so many methods to subvert such a restriction, and most of them can't be removed

That would not be possible because Macs are much more open by design. There are so many methods to subvert such a restriction, and most of them can't be removed without significantly degrading the utility of the OS. And even if we suppose they did do that, users would just stop updating and stop buying new hardware incapable of running old OS versions.