Only when MS allows booting "vetted" non-MS OSes _with the same amount of effort_ as it takes to boot MS OSes (like what they do already in other devices) there

Only when MS allows booting "vetted" non-MS OSes _with the same amount of effort_ as it takes to boot MS OSes (like what they do already in other devices) there is any minimal chance of believing this is for security, and not for pure market control. If they wanted to have a setting to control whether to block non-MS OSes to boot, they could have put THAT as the hard-to-find option, rather than the opposite. They would not have needed to make an entire class of separate hardware whose only difference is apparently that by default it boots MS OSes only, and who knows how is being pushed behind the scenes.