My buh was for suggesting it was inefficient in single-user systems. Personally, I like the fact I can still web browse without lumpy interruptions while a CD burns or rips, or when my mail client checks for new mail or usenet posts, or when I'm encoding my recent holiday videos, rasterising a printout in the background, updating the search database in the background, etc etc etc.
I'll think you'll notice that 1GHz ARM-compatible systems are already available, and more are on the horizon. Additionally, Linux on ARM systems, even ones such as the A9 Home, have much greater throughput than RISC OS does; precisely because of its PMT and better memory management.
There's no point porting the OS and rewriting the API at all, you're right. The amount of old software that would break in the name of progress will massively outweigh the new software that might be written, and most people would stick with their ancient crumbling RiscPCs anyway.
(Which is precisely the situation that let me to use other systems for my day-to-day computing.)