Interesting observation, Ray.  I agree that the buildings were very busy, but I also found the fleamarket to be bigger and more vigorous than in recent past years.  

Newer hams seem to be naturally more attracted to new equipment than retro parts and used gear.  There is also much interest in particular modes or activities that are relatively new like FT8 or POTA.  The Hamvention markets itself to newer hams and lets the older crew go unaddressed (probably assuming we will come anyway) but of course we gradually stop coming as we age and die.  All this combines to produce the results observed.

I suspect that the interest in old gear may rebound to some degree as the next generation steps up.  But one limitation is that the modern gear of today is somewhat more difficult to repair, especially for beginners.  Part of this is the micro-miniature, surface-mount technology, but perhaps an even bigger issue is missing or limited-quality service documentation from some manufacturers.  This leads to a greater (but understandable) fear of buying used equipment.  And this will limit the hamfest fleamarket of the future to some degree.