If the Class E above D is a measurable safety problem, why does it work so well in the USA and Canada – without even a mandatory transponder requirement below 10,000 feet in enroute airspace, and with over 50% of the 350 Class D airports in the USA not having radar coverage in the E airspace immediately above the D?
Dick, I and others have tried to explain this before; I'll give it one more go.
E over D is not as safe a C over D; fact.
If you increase the risk, under the ALARP and modern risk management processes, you must have an identifiable cost benefit.
ie increased risk is ok if it pays it's own way.
It isn't about E over D is not safe; it's less safe (in that airspace, leave the CTRs out of the argument).
The USA probably wouldn't have done E over D if they started with C over D first; but they didn't. To simply say they do it so we can too is simplifying the argument and not addressing our legislative requirements.
If you change our laws/regs then it may be feasible to do what you suggest; but until that happens we must follow our laws.
E over D is still possible; even with our laws, you simply have to prove why the cost benefit is viable; that's the hard part.