fujii,
I suggest you do a little more homework.
Of course ATC provides a separation service, but it is the standard of the service, the CNS/ATM resources devoted to the job, that is determined by the separation assurance standard.
One of the worst things that was ever done was to name airspace designation categories Class A through G airspace, because almost universally, and very self-servingly by domestic airline pilots in Australia, A through G are treated as "safety" levels, which they are not.
The ICAO risk management approach is that ALL airspace is equally "safe", ie: the risk doesn't materially change. The separation assurance standard is maintained at at least the generally quoted minimum level of 5 in 10 to the minus 9.
As traffic goes up, separation services have to go up, to maintain the standard.
Put another way, you have to increase ATC resources to handle greater traffic, that is what ATC really does, enable greater traffic levels in a given volume of airspace.
If you care to delve into the Eurocontrol web sites (lets get away from US for a bit) you will find their target separation assurance standards, and unlike Australia, they publish quarterly reports of the actual results achieved --- that is, they reduce loss of separation events to present an achieved result (probability) to compare with the target separation standard.
It goes without saying, but few domestic pilots in Australia accept it, but providing more than the required resources (ie C over D rather than E over D) is economic waste, as the separation assurance standard is already so high, the probability is so low, that the additional resources do not decrease the collision risk probability. C over D means you are increasing ATC resources as the risk decrease, ie: the traffic spreads out away from an airfield.
But they do give a certain cohort of pilots the warm and fuzzies.
While I disagree with the CASA methodology for doing airspace assessments, they are aiming at a risk management process that is intended to achieve the above result ---- are more or less ATC resources needed at a particular place, or put another way, does the airspace categorization go up or down or remain the same, based in current traffic levels, each time a review is triggered.
I would also suggest the above constitutes fact, its existence is not a matter of opinion.
Tootle pip!!