We did used to see roughly one aeroplane per year destroyed by PPL(A)/CPL(A)/ATPL(A) holders declining conversion training. So, some form of mandatory conversion I think was necessary. But, treatment in the same manner as, say, a tailwheel or VP conversion would have been more appropriate than the current muddle.
Incidentally, a microlight PPL allows you to fly both 3-axis and flexwing, twins, variable pitch and (if any existed, which they don't in the UK) retracts without any additional training. Not very consistent is it !
G