As I've said before, the "complex" definition is mostly a historical artefact and barely relates to complexity in any real sense.
Modern avionics are a lot more difficult to grasp than a CS prop etc. - and far fewer instructors (in the UK, for sure) understand avionics. When I got my TB20 I never found an instructor who even know how to use an HSI. I worked it all out myself, zooming around at 5000ft over Kent
It is however true that anybody not totally dense can learn to fly a C152 on nice days, but IMHO only a fairly small % of people can fully grasp a complex panel and use it fully for IFR. I know my plane pretty well but there are still a few little nooks and crannies in the GPS which I have never used (and I avoid them in the air; they are not operationally relevant).