If you build to the new release, there's a good chance it's not going to work well on the next one though. This is part of why Windows Mobile failed. There weren't that many apps to begin with, but you needed to build differently for WP7, WP8, and WM10 if you wanted things to really work. And WM10 never got any users.
They really need to just figure out how you make a GUI app on windows.
I want to make a GUI app in the same framework the Office team uses, if its good enough for them then good enough for me, see "eating your own dog food".
Despite the appearance of being separate processes because of separate windows, excel is basically a detached MDI interface where each sheet gets its own window, but same process. I think the other Office products are the same (but I'm not sure). I dont know how many hours I wasted at a previous employer trying to figure out what was wrong, but finally gave up after many frustrating hours.