Windows RT didn't fail because of ARM
It failed because Microsoft hobbled the OS, cutting away useful features in order to justify the markup on full-fat Windows. We now see how that worked out, with Microsoft having to give away full fat Windows to attract hardware manufacturers, RT dead in the water and Microsoft only able to compete in the tablet space at all thanks to Intel giving away their x86 chips at zero cost (or worse).
Microsoft botched Windows RT right from the get-go. It was a poor cousin to a piss poor OS (Windows 8), and stood very little chance.
A decent version of Windows on ARM could have been a very fine thing, however the fact it wasn't had nothing to do with ARM and everything to do with the crass and arrogant design choices made by Microsoft.