PPL/IR would probably like universal Mode S
I don't think that is at all true. Mode S has exactly zero value to anybody in light GA, over Mode C.
Even transport jet TCAS gets no benefit from GA targets having S, over C.
Some ATC units in the "white christian" Europe use Mode S to help jet traffic management, and e.g. NATS have some fancy software for holding stack display and management, but light GA doesn't go anywhere near that airspace. Even a PA46 or a Jetprop doesn't go into the same airspace.
Mode S would be worthwhile if one got TIS uplinked over it but that won't happen because nobody is going to pay for it.
What would make sense is mandatory Mode C - on all aircraft technically capable of carrying it. But there are so few midairs that nobody is going to do that one either.