I roughly agree with you but this hypothesis would suggest:
- more helmet wearing in less cycling-friendly cities like Rotterdam which, IIRC, is an example of such a place in the Netherlands
- more helmet wearing (in the sense that the ratio between Dutch helmet wearing and helmet wearing in other Western European countries is higher) at points in the past when bicycle infrastructure was less protected. Though this is confounded by lots of things.
- more helmet wearing in less cycling-friendly cities like Rotterdam which, IIRC, is an example of such a place in the Netherlands
- more helmet wearing (in the sense that the ratio between Dutch helmet wearing and helmet wearing in other Western European countries is higher) at points in the past when bicycle infrastructure was less protected. Though this is confounded by lots of things.