Oh, and having PMT under RISC OS would really screw things anyway. You've have to make lots of changes that'd make current software incompatible. (In an ideal would, they would have started to make the changes at the same time as releasing RO5, where software had to be changed anyway.)
There are lots of things in RISC OS that there are only one of. Like the current directory, OS_Plot contexts. FileCore isn't re-enterant, lots of current programs didn't have their messaging code designed for PMT, etc. All sorts of reasons why it'll be an arse.
People'll be asking for more memory protection next, and then they'll complain everything's so radically different, nothing old will work at all.