But if somebody tried serious IFR in a 1970 C150 with its single VOR receiver about to fall out of the panel, the pilot workload would be, shall we say, rather high. Not everything can be regulated and some people cannot accept that.
But to be fair, in the USA many people fly old C150's IFR in airways, shooting NPA's all the time and they don't make a big deal out of it....because really it is not a big deal....I've flown some dodgey old wrecks IFR with MEAs up to 11000 and never once had to tell ATC I couldn't make it. Sure it is more work than direct-enter-enter but once you get used to doing it, it is "fun" in a strange sort of way
In Euroland we like to make it a big deal, which is why the JAA IR has become so sufficiently gold plated that no normal PPL can justify doing one. Equipment is another issue. In USALand, you can fly your experimental IFR (Equv. of UK permit aeroplane more or less). You can fly your ex-mil jet IFR, and even shoot a GPS approach in it with suitable kit. In Euroland they like to gold plate the kit you need too, which doesn't encourage private IFR (E.g. my aeroplane has a VOR, NDB etc....but I can't fly IFR because my radio is not FM immune.....and my GPS is not IFR certified and I have no DME).....In the states I'd be perfectly legal to fly IFR, shoot a VOR, LOC, ILS or Back Course approach......