I think I'd go for the fatigue argument. I was on the penultimate 2 FTS (Syerston) JP course (graduated 2nd half of 1969), which IIRC had the last Mk 4s on BFTS duties. For curious reasons (tinkering with flying training syllabi!), some of us only flew the Mk4 (no 3s or 5s), and I can assure you it was an interesting transition straight from the Chipmunk
Two things I can recall about the Mk4 fleet there: first it was shrinking because a number of "bitter and twisted" jets came back off airtest to be scrapped, because they would not stall straight - the fun ones would even flick! Secondly was an aircraft so fatigued that it was "red lined" (not that I knew what a red line was then): "No aeros, navexes only, one landing per sortie." And they let frightened teenagers
off solo in that one!!! ..... and yer tell that ter the young folks today .......