This is exactly what I've been saying they should do since the launch of the first Surface RT.
No it isn't. THIS is what you've been saying.
Windows Phone OS (ARM) for Tablets under 9" and full x86 for everything bigger
Artificial device size restrictions have NOTHING to do with a unified store, a merged ARM OS, or unified APIs. What if I want an 8" ULP x86 device? What if in the future ARM devices run all the apps I want (realistically they're almost there now if you don't need legacy support)? You could get a 10"+ ARM-based tablet for cheaper and with better better life, for instance.
With a unified store, and soon unified APIs, more and more apps will be available on both ARM and x86 through the new store. They're trying to UNITE the architectures, so everything runs on both. What you're suggesting they do is divide them, and put artificial limitations on which runs on what.
I've seen no evidence that ARM chips are superior to the current crop of Intel and AMD chips in anything other than the smallest of form factors (smartphones). All of the benchmarks I've looked at are showing Bay Trail and Temash destroying anything with A15 cores (Tegra 4 for example) in performance and matching it for power consumption. Why would you want to be limited by an ARM chip if there is no tangible benefit? Cost? Maybe in the absolute low end, but Window's tablets haven't been playing there due to the RAM and disk space requirements.
As for RT devices being "up there" I think the market has proven you and Microsoft wrong. Not a single other OEM is offering an RT device. Why? Because RT is garbage in its current incarnation.
Use ARM where it's best for now (phones) using a more optimized OS like Windows Phone 8 instead of trying to cram full RT onto small tablets where the desktop can't even be utilized properly. That's my point in size restrictions. No desktop part on sub 9" tablets and phones, put the desktop on the 10" plus where it can still be used for productivity. Where the desktop is included, keep it to x86 for desktop apps.
As much as Microsoft doesn't want to admit it, the desktop isn't going anywhere soon. People doing actual productivity want re-sizable windows and multitasking capability. Even Android manufacturers like Samsung are seeing this and are developing multi-windowed modes.