For goodness sake Dick, this is the 2015s, with great FMS nav systems and EGPWS. You don't base a complete airspace system on stopping aeroplanes running into hills! ATC's purpose is to keep aeroplanes apart.
Agreed. These days, even a 2 seat ultralight has pinpoint navigation accuracy and terrain awareness from cheap, tablet-based programs such as OzRunways.
Keeping aircraft from getting too close, in AND outside CTA should be the goal.
PG