Pace,
I think you just confounded a bunch of things. Yes, the safest thing (whether VMC or IMC) is to be IFR in CAS all the way.
But very often that is not an option. The two obvious exceptions being VFR airfields and IFR airfields outside CAS.
I don't have any stats, but I would suggest that the proportion of flights that an EIR makes, which by definition are V/I/V, which never leave controlled airspace are very small. Most EIRs will be based in Cumbernauld, Popham, Blackbushe, Biggin and so on. There will, of course, be some at East Midlands and Teesside, but they will be the exception by, I suggest, an order of magnitude.
Once you accept that most EIR flights (and GA IR flights for that matter) will have an OCAS element, and a substantial proportion will have a VFR airfield at one end or the other, a lot of these theoretical objections begin to evaporate.
If I were to characterise the EIR flight, it will be from Blackbushe to Troyes. It will be executed exactly as such a flight would be executed today VFR, with the same eye to weather, except that it will be flown in comfort and safety at F090 for the bulk of the flight, and only the two ends will be scrabbling around uncontrolled, low level VFR, just as now.