Interestingly enough, this has been done before! Windows 7 introduced "Windows XP Mode" which was a container running a virtualized Windows XP for applications

Interestingly enough, this has been done before! Windows 7 introduced "Windows XP Mode" which was a container running a virtualized Windows XP for applications that didn't work correctly in 7. But since they also had the per-application compatibility settings, it wasn't the first pick for most users. While it's a good idea on the surface, it still means they have to maintain compatibility code within that virtual system. Perhaps if they separate it well enough, and it has a dedicated team, while the core OS team can focus on a slimmed down, "current stuff only" version... but I suppose it's also hard to know where the line is drawn in the sand.