Maybe by comparing the comparative cognitive of twins, one of whom has a hobby and the other doesn’t?

It'll definitely be a nice data point.

It seems also just enough to say supporting people to have a hobby if they're at risk of dementia and don't have one is important to meeting their social and health needs.

I might be being too cynical. I wonder if there is some funding arguments, I.e. if it's for their health and insurer may fund it.

If it's for their social/cultural and general wellbeing it might be out of their remit somewhat.