The hard part for Microsoft is not the porting of Windows to ARM. The hard part has been and continues to be convincing existing Windows developers to port their apps to UWP.

The reason Windows on Mac is useful to people is because people have may programs that do not have acceptable equivalents on Mac OS. These apps are usually legacy programs that probably fill special niches, and the cost of "modernizing" is usually not justified. This effort would be required to port these apps to Windows ARM, which last time around also required porting to UWP which Microsoft is still pushing, but not easy to actually do for a lot of code bases.

A new Microsoft irony kicks in here is that if they are successful in convincing the huge Windows ecosystem to migrate to UWP away from Win32, et. al, they may actually kill their lock-in advantage. A serious rewrite at this stage may also invite Mac, Linux, iOS, Android ports. In this case, Apple no longer needs to care about Intel Windows and this bullet point becomes less compelling and maybe they could reconsider.