This is very hard. Supporting API + UI changes is not easy. Microsoft is not doing it (Skype etc) so why should small teams do it. We don't just program we run BUSINESS.
You see the impact on a few things like Office and SCCM today. As the legacy windows platforms fade, it will become a thing in other areas. My guess is you’ll see more “fixed in Azure only” scenarios to drive demand for those products, and act as a sales funnel for complementary services.
It takes less time to test compatibility than it does to compile.