Bear with me on this. I think you're saying there is no point in rewriting the OS for PMT because there old software will break and there is no new software, and with such a small dwindling userbase, there is no margin in writing new software.
The only way around that is to increase the user base, for example by porting it to new, cheap, hardware. If that works, it might be worthwhile writing new software and if it is properly written, it will work on either PMT or CMT.
With regard to the ARM and multithreading, again we are using the same terms for different things. The ARM does not contaibn visible copies of the user register set and this would be required for hardware multi-threading. I hadn't really thought about the SWAP instruction, but it might tip my view in favour of PMT.