ARM makes so much more sense here than for the Pro X. Maybe needing x86 emulation, and Qualcomm's pretense to being ready for the high end, pushed MS to getting a beefier chip and so a pricier device.
ARM is still in the period where it's making big strides each gen. I suspect the right Surface gets built eventually, once folks who aren't QC can license some huge-core ARM IP and fab it on a good process, so you get cheap ARM chips that can take on cheap x86 ones in computer-y settings. It is somewhat disappointing right now 'cause all the parts are there, but haven't quite been put together the ideal way.
I'm sure more things get ported, too. MS being a cloud provider, I wonder a bit about ARM devices streaming huge apps (Visual Studio, games, etc.) run remotely as another way to sidestep their legacy troubles.