I agree with Peter (flibble). Unless the newer operating system can do something for the programmer that an older one cannot (which could include saving development time), there is no reason to limit code to the newer version.
The same goes for hardware; in fact, if I found myself using using FireFox regularly on my RPC, despite it running like treacle, I would probably go for a faster computer. Since it doesn't run well enough to be practical (note, I don't mean fast enough, I'm quite patient), I can't see anything that an Iyonix or A9 can do for me
RISC OS software is somewhat hobbled by the fact that it tends to be written to do a particular job well, which means that it's rare that a new version of something will have existing customers clamouring for an upgrade, but would you have it any other way?