You are right, on the other hand, Microsoft isn't famous for being able to keep up with itself (especially if you look at the number of deprecated ways of creating GUIs).
Depends on which "last 4" you are talking about. UWP's GUI API is essentially the completion of the "Avalon promise" from the Longhorn/pre-Vista era: WPF and Silverlight (aka WPF/E) were both "spin offs" from Avalon in their own ways and WinRT was final push to complete "Avalon" before being rebranded UWP... So from that sense Avalon, WPF, Silverlight, WinRT, UWP are all iterations of the same API plan, all share more similarities among each other than dissimilarities, and all share a base set of transferable skills that learning one of them makes it easier to transfer to a different one. So from that perspective too, the "last 4" APIs they told people to use have all really be just one API that's taken a while to grow, evolve, then re-converge.