They're purchasing the OS, not the CPU/SOC architecture.
Microsoft doesn't have the same loyalty that Apple does. They can't afford to release an ARM OS that isn't already supported by all major software applications, and majority support for normal x86 apps, with assistance and roadmap to completely bridge the gap.
When the transition is seamless, or 90% seamless, the architecture won't matter, and customers won't even realize they've switched.
If they release ARM hardware that doubles battery life and performance, but doesn't offer a seamless transition, it'll flop. Just like their last attempt did.