The whole basis of the Chicago Convention, of-course, is based upon the homogenous safety of an aircraft + flight crew licences + ground crew licences + operating procedures combination.
And in 1944, that did make a lot more sense than perhaps it does now when global equipment and training standards are a lot more universal. What makes it even more daft in Europe is JAA and now EASA which has created uinversal standards in many things, but it remains the case that each national authority is still lord of its own airspace.
Instrument flight is probably a latecomer for necessary global convergence. As I understand it, the "vanilla" FAA IR is rather closer to the UK IMC - which since US airliners aren't routinely embarrassing themselves around the world presumably means that there's additional training in the ATPL and/or type rating courses for the airlines. Fair enough.
In the meantime, I learned the other day that Transport Canada has "VFR Over The Top" or VFR-OTT rating, FAA has an IR which is affordable and sensible for a PPL, the UK has the IMC rating which is also affordable and sensible for a PPL... Even the French are now talking about something that looks like an IMC rating.
... we aren't going to change the ICAO convention requirement to match licence with state of registry.
So what we need is an ICAO IR for PPLs that is acknowledged across the world. The right solution to this is probably that the IR in Europe, and I'm sure one or two other places, is trimmed down to the ICAO minimum (which will then look rather like the FAA IR), and additional material needed for the airlines is moved over to type rating training and tests where it really belongs.
Maybe this would be a worthy project for IAOPA?
G