RISC OS already runs on some ARM9s (Pace had a demo about 3 years ago, Castle/Tematic mentioned some porting work for another customer). But if the processor is, say, 5% of the cost of the machine who's going to want a slower processor for a $10 saving in machine cost?
If you're making a Gameboy or a mobile phone, this is another question. Though if one of ARM's licensees comes out with a processor significantly faster than the i80321 with the right peripherals and at the right price, I'm sure RISC OS hardware manufacturers would jump at the chance.