I don't completely agree with the assumption: -
'...but where the issue is conflict between unknown VFR against IFR traffic on an instrument approach, class E completely fails to resolve this'
I'm not suggesting that its not correct, it is, but with a Class E zone shown on a map, I would imagine the vast majority of users would call the ATC Unit in question anyway. You'll still get the odd glider/military/non-radio/luddite but the enlightened amongst us will realise that you can get a sensible and relevant Air Traffic Service, free of charge when transitting a zone!
The problems of the perception of CAS are more fundamental, IMHO. There is a certain element amongst the GA community that really believes that any CAS is a no-go area unless you're in a 737. They don't understand the subtleties of airspace classification or how to 'use' the service. Furthermore, abyssmal RT standards mean they're either overawed by the prospect of attempting a transit or simply too cr@p on the radio to be trusted to transit! Then you get the arguments that 'I shouldn't have to use the radio to fly in my xxx type from Upper Lower Little Snodbury to Wotsit under the Over'. True, as a PFA flyer, I enjoy the true freedom of Class G too, but it's reassuring to know that there's a wealth of info and assistance available to me if I choose to use it. Nobody bats an eyelid about drivers having to learn the Highway Code before being let loose on the road, why do (some) see the RT test as such an imposition?
The problem isn't just with pilots. There are certain ATC Units, who operate their Class D Airspace almost like Class A. I've had some very odd zone clearances before (hold here, orbit there), which I can envisage in plan form on the radar screen. The simple fact that I can see your ILS traffic and will easily avoid him if he goes around is overlooked and I'm afforded a 'Separation' that I don't actually need by default. TCAS and Mode S don't help these scenarios either. This problem will get worse, particularly as NATS feed their new ATC recruits straight into Approach Radar, with minimal instruction or expereince of either GA, VFR or the dreaded mythical world of Outside CAS!