Interesting subject
Just random thoughts really
There was deff a massive surge of interest in retro around the period I joined 2008/2009 everyone wanted a retro bike . That fad seems to have gone for now leaving the real fanatics, I think age is creeping into it now too for example at mayhem in 2009 there was still lots of riders still riding that rode the bikes we were on back in the day this must becoming less and less now.
It has been said before but I also think people do move on , most of the original people I rode with when I first started are still riding but not retro or very little retro. Me included!!
If we are looking at the national ride turnouts I think it's more to do with money , these days I couldn't even think about driving 500+ Miles upto the lakes and booking b&bs for a 20 mile bike ride. Didn't even question it 4 years ago