If you go through the AAIB reports you will see that most mid airs are low down. The last UK one I remember being higher was the 2007 incident which was at 1400 agl 1800 ft amsl. However there have been mid airs in the US at 16000ft (glider / Biz Jet). On a Saturday in summer with good soaring conditions there will be a lot of gliders at 2000 – 10000ft not transponding. Modern 3 X micros are also likely to be at “normal” GA levels.
I do not subscribe to the theory that collision avoidance systems make you complacent. We know that about 50% of the potential threat will not show up on them yet they spot several times the traffic that you pick up with see and avoid. If anything this makes you a lot less complacent.
Rod1