I’ve never seen that decline associated with the British Invasion before. I’ve always heard that the cause was kids not getting started, mostly due to the emergence of video games in the 1980s.
Stamp collecting was dying when I was a kid 50 years ago. Coin collecting was ruined when it became a big business and your chances of finding something in circulation became dead zero. Electronics as a hobby (as in building and programming otoh is very educational and creative. You can do things now for 20 bucks in a week on a kitchen table that would take a team of engineers working for a substantial company with thousands of dollars of equipment 6 months to do 35 years ago.
Lego model building is going pretty good.
It is to the point if you want to do big custom designs of your own there are websites you can order the pieces you need to do the project.