Ian,
With respect, I would make the following comments:
A good question - it is a common problem that the actual data is too thin to analyse normally. We have 40 years data but when split into decades there are too few MACs to get a smooth curve.
In fact, the sample is so small that we can examine the circumstances of each MAC --- and we have, and there is no common trend, each one was unique. In my view, attempting to draw any conclusions from smoothing the data does nor produce a valid outcome.
What I have already alluded to is "other factors" that have become evident in the last "9" years (MACs are only party of the story) that have resulted in some disturbing trends ---- beyond, of course, the fact that GA is dying.
There is no sharp cutoff. One only needs to decide on a "relaxation period" where the technology has changed a certain amount,
Except for one problem, there have been
NO significant technology changes, indeed most of the GA fleet is 30 or more years old.
A new C-172 is just an old C-172 (including the grandfathered certification standards) with Garmin glass added, lovely stuff, but it doesn't change how you fly around a VFR circuit --- or shouldn't, and in any event there are only a relative handful of such aircraft in the civil fleet. There is no shortage of data on fleet average age.
A rigid fact of life for many flying schools is that the only affordable replacement a for a 30-40 year old C-152 is a rebuilt 30-40 year old C-152--- or a PA-23 likewise.
There are aircraft in daily use today, that I first flew "out of the box" 40+ years ago. Despite comments in another post, I don't believe pilots are "head down" mucking around with GPS in the circuit, and GPS is about the only major piece of "technology" that you could call new.
The training syllabus has been mucked around with in that time, for little benefit, but the core syllabus has not really changed much since the mid-1930 --- but how the volume and style of regulation has changed.
There has been significant technology change in the above period, as it effects air transport, both aircraft and CNS/ATM,
but not General Aviation.
A mass of historical data is available, Bankstown, Archerfield and Moorrabin are quite places these days, compared to the mid-60s to the mid-70s.
Tootle pip!!