Pringle,
I don't think there would be many ex airline pilots currently working as sim instructors who will try to take the jobs you refer to. Currently a sim instructor teaching on a level D sim in the UK can earn between £500 - 1000 per day (if they are prepared to do two sims in one day ie about 15 hrs work). Given the choice of doing that or bashing around in a seneca I know which I would choose.
However, there is also perhaps another point which should be considered. If an ex airline pilot with say 15,000hrs of commercial IFR experience is allowed to teach IRs on a PPL, might that experience be useful to the flying school and the students, possible more so than a CPL/IR with the minimum IF requirement to be an IR instructor. Whilst there must be a career path for instructors should the needs of the students also be taken into account.
I am not, by the way, suggesting that all airline pilots would be good at teaching IRs just because they are airline pilots, or that CPL/IR instructors wouldn't be. However I know that when I instruct instrument flying nowdays for IMC ratings I do a much better job of it having spent the last 15 years flying multi-crew IFR than I used to when I had a CPL/IR and very little actual IFR experience.