So why should we think it acceptable for people to train in non FM Immune, Mode S etc aircraft?
I don't think you actually meant to say the above, bose-x, because a) there is no evidence that FM Immunity has any practical meaning (it is a theoretical device to protect the aviation band from the near end of the FM band, and assumes rather poor selectivity of the airborne equipment) and b) the case for Mode S for GA OCAS (where most IMC training takes place, mostly in VMC) is awfully thin.
The instruction standard is patchy but this is because the PPL training business is for the most part decrepit (due to lack of business) and cannot afford to get instructors other than ATPL hour builders and others on peanut pay, who in most cases cannot afford to keep their IR (assuming they had a SE/non-MC IR originally) valid because it costs too much to do so, relative to the returns. There are plenty of instructors with an IR but when you quizz them you find most of them are lapsed. The hour builders tend to renew theirs only when they get close to getting a job offer.
I think the IMCR is OK for the privileges, which are mostly exercised in Class G, but there isn't really a solution to this problem, because no matter how you train people, they will still be stuck on the aircraft and the budget, and no amount of training can help there.
In the hypothetical case of a (say) £8000 IR (50hrs) replacing the (say) £2500 IMCR, all one would achieve is a very very low take-up, because the CAA isn't going to be handing out nice new planes for people to fly, and subsidise their operating costs so they can stay current.
Like it or not, an IR (JAA or FAA) is largely a game for highly motivated and well funded owner pilots and much as many (I am not saying you in particular) like to think that a new IR will improve the general GA IFR situation, it won't ever reach the PPL masses.
What will eventually improve GA IFR utility will be the gradual scrapping of the junk sitting around airfields today and its replacement with modern (DA40, etc) planes with decent IFR avionics, autopilots, and all the other stuff that makes IFR flight realistic. Somebody flying one of those will be fine even with dodgy training, whereas if flying some piece of junk with half duff avionics one could have an ATPL and it won't do them much good.