Sorry, quote and bold did not show so...
Thanks for making the key point: the cycling-as-recreation riders (MTB*, roadies**, BMX, tourers) who were mostly already wearing helmets continued. But those who stopped (almost by definition short trip utility riders), heck, they don't count. They're not committed enough. etc etc etc.
To return to your opening quote, the point is that MHLs effectively curtail the adoption of cycling for ad-hoc short distance transport. That is, getting from A to B. Without activity-specific clothes. Without a shower at the end.
For those who do want to refer to some data nothing shows this more clearly than the fate of MHL bike hire schemes,which specifically target the casual, short distance rider.
Yes, while people who used a helmet pre MHL may have found the transition simple, it is not commitment and just BS to say they now and always will use one when on a bike now in Aust or when OS. Plenty of riders who did not use a helmet in these groups just got on with it and started to use them. Some do so only for comp and some who did not continue, to do so.
The groups you point out is just an attempt to make tribes which really is just daft and very trump like. People come and go and move and then disappear from all these "groups" and other biking methods. ( Worth noting that many of these "committed" or comp type riders as you class them are now riding on what they call gravel type bikes, on gravel type roads just to get away from roads and traffic. Maybe just me but I have seen plenty who then ditch the helmet from the head and strap it to the bike in some way. Once back on the tar and in traffic the lid goes back on)
If you need to see an clear example of these three points you noted working in the opposite, go and spend some time in the ACT. I regret to advise it is in Aust and is largely made up of people from other states, they may stay for a while or then move on. I would say it has a very good mix of all things bikes and what you can do with them and yes the bike hire scheme is still active and works ( well it was at just before stay at home/ lock down rules came into NSW and the ACT ).
And a question for you. Do you really think to keep highlighting riding in Holland vs. Aust is even worth the effort? Ok the population is more or less down a few million on us but they are about 65% the size of Tasmania and really, all other of a similar size and population to them countries struggle to keep up with what they have done bike wise. I say this as being half Dutch and I know they would think this is very, very odd.
Anyway I just noted the ABC www is pointing out new riders are on bikes in quite a big way. Is and was MHL is really stopping these new people starting or was it really the lack of basic infrastructure in combo with the fear of the hostile roads?