Um, I'm not sure what you're talking about. First off, Windows RT for all intents and purposes is dead. This is mainly because Intel finally got around to creating decent low voltage processors that could compete well with the ARM processors. The new generation of Intel Atom processors have done very well in terms of performance and battery life, and can run the x86/x64 code.
Just because of the newer API doesn't mean their getting rid of the desktop. Quite the contrary infact the first version of office for RT ran on the desktop not in metro. The idea for the newer API is so that there is a common code source so they can offer apps that will run on a tablet or desktop or whatever.
The Surface Pro 3 was the hottest selling device of 2014, with an update due out in 2015, and it was the first Surface devce to really take a chunk out of the iPad and Mac Book air market. It's a full i5 or i7 device, and the bakers have been testing Sonar with it. That doesn't mean that it's not going to work with your desktop/workstation computer, it's simply that the market is changing. Think of the Surface Pro 3 as a Toyota Prius, where the standard Desktop is a Toyota Tundra. They are both designed to move people and things to one pace or another, but if you need to put on a tow hitch, you're not going to do that on the Prius.
developers are going to look at the Windows store, because it's simple advertisement. How much work did Sonar have to do in their past to advertise their product, going to a webpage, and manage google search. Now someone with a Windows 8 tablet, laptop or desktop can go search for DAW or recording software, and they will see a link to Sonar. Right now it just has a link to the Cakewalk page, but if CW can offer the download direct from MS and it's a win. Why do you think the bakers went to so much trouble getting Sonar on Steam?
This is the new marketing model for computer software, like it or not.