(I'm going to exclude Linux in the below, because it's an odd duck where users/distributors can reasonably be expected to recompile every program they use for a new architecture.)

Sure! But that code is generally core OS code written by a small number of people and deployed widely. If Apple or Microsoft or Google decided that OpenPOWER was the way to go, we'd be able to switch over at least some use cases. ALL of the above have current Arm offerings. Arm has reached "good enough" performance - the snapdragon 855 is as fast as a i5-8630U, and uses half the power.

The primary barrier to processor migration is compatibility with user-mode programs. Since the vast majority of programs are now using primarily ILs, such as JS or C#, once the runtimes are ported (and they have been), there's not a lot of lasting incompatibility since programs bind to APIs, not ISAs. Apps for Work (CAD, Photoshop, dev tools) are traditionally the last to port, but there has been no better time to switch architectures than now, and it's only getting easier.

As someone going through a processor migration right, now, the core OS is actually not that hard to move. But even once you change all the language runtimes, there's a fair bit of proprietary software (including games) that are unlikely to move quickly.