If the hardware vendors go "kaput" there would be little point in keeping RISC OS going would there? I can rattle off a few "emulated" systems such as CBM64 or Sinclair Spectrum - but really you can't exactly describe them as "alive" when they're just curios to show off to people in order to prove "look how fast my PC is it can emulate xxx"
I also note you say "hardware platforms" - why the plural? I don't distinguish between any specific make of RISC OS hardware. The availability of a "native" platform (whoever makes it) is what counts. That having been said Open Sourcing RO5.xx probably benefits - in the short to mid term - Iyonix users more than A9 ones - simply because it's the OS Iyonix uses. Over time though there is no reason (other than time, effort and man/woman power) to see RO 5.xx appear on the A9 (or on other ARM platform).
The RISC OS (IMHO) only has value while it's a real OS running on real hardware - the moment it becomes an "emulated environment" on some Windows based platform it would probably become time to pull the plug... no doubt others will disagree with that (as is their right).