"Over the intervening years N reg aircraft have popped up everywhere in Europe and it now appears to be a semi global flag of convenience "
Or the choice of discerning pilots who chose to avoid the worst parts of EASA oversight and take advantage of the ease and pragmatism of the USA pilot certification process? Convenient? certainly. Flag of convenience - not so much. The safety standards are just as high, but the attitude to general aviation is somewhat more helpful. For a start , anyone on a N reg has avoided the UK CAA to JAR to EASA to Brexit unknown saga.
And what about those N reg aircraft that belong to US airlines ? also popping up on a regular basis.... So where is the difference?