With desktop Windows on ARM, they would have OK battery life, but few apps. With desktop Windows on Intel, they would have apps, but no battery life. With desktop Windows on either architecture, they would have very little support for normal phone use cases; interesting as it might be, it doesn't sound like a recipe for mainstream success.

Edit: in fact, I suspect that even on ARM, without heavy-handed optimisation for battery, it wouldn't have good battery life.