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.
Still a lot of reason to wonder if they had found a way to deliver the UWP platform earlier to Windows 7 ahead of 8's launch and got more applications up front of the transition if that attempt might have gone much smoother.