FFF
I think you hit in on the head with availability of suitable aircraft. Come to think of it, the IMCR holder has the same problem: one needs FM Imm. for IFR in Class D too. Not a lot of people worry about this
In fact there is a widespread general problem with availability of aircraft suitable for any IFR, never mind Class A. You more or less have to buy one.
So, IMV, the group of people that would do a Euro IR would not be a lot larger than the group that is/would do the FAA IR. It would be the same bunch of owners / part owners as are going N at present.
IF the certification issue was fully resolved (which basically means EASA accepting FAA certification without ANY Euro based certification firm skimming money off the process e.g. in issuing "Euro STCs") then the two should be comparable; in fact the Euro IR group might be bigger because of no need to put the plane on N which is itself a massive hassle.
However, the above is unlikely to happen. So, we will have
1) Anybody with an already-N plane will stay FAA
2) Anybody with a plane that HAS to be on N will go/stay FAA (e.g. SR22, various higher-up stuff)
3) Anybody who cannot pass the stupid JAA audiogram will go/stay FAA
If the Euro IR is in any way crippled in IFR privileges relative to ICAO privileges (e.g. the aborted and bizzare IAOPA proposal for a Euro IR limited to 10,000ft - how could any PILOT come up with that?) the uptake of the Euro IR will be minimal
Finally, I don't see the IMCR losing out to a new IR because for piston pilots the IMCR is good enough for the UK. An IR is 99% for European touring.
There is a lot to do to make this work. Not just cutting down the ground school.