I wonder if those middle levels are still as deserted away from the major terminal areas??.Its still a very big sky!!!.
On a flight across Europe, FL100-180, say 700nm, you are highly unlikely to get visual with another aircraft (not counting contrails at FL300+ etc). And this includes crossing the busiest areas e.g. Brussels.
"IFA GA" flies in a huge empty void.
The only busy bits are the terminal areas but they are dealt with by minimum overfly levels (say FL130 for Frankfurt) when you occassionally get visual with big stuff way below, or by Eurocontrol routings taking you way away from these areas.
The radio traffic confirms the situation to be as I describe.
As Cobalt says, very little VFR traffic above FL060, in fact I would have used a much lower figure than 060
I'd say 95-99% is below 2000ft.