A very interesting discussion. Here in the UK the crowds at Club meetings definitely seem smaller even allowing for rose - tinted specs etc.
Modern TV coverage, let alone other mobile media have changed sports viewing interest generally. Take the UK soccer Premiership, immensely successful but live attendance revenue is now only a small part of its revenue. Look at the pitch side adverts and you will see many in Chinese. It's probably fair to say that 90% of Man. United or Arsenal fans will never ever go to a live match partially due to ticket prices.
Similarly our last US visit coincided with the start of the NFL and College football season and the coverage was overwhelming - ten TV channels of it, huge screens in every bar.
So don't judge motor racing by trackside attendance alone, that is SO 20th century (!).
As mentioned above club racing is really for the competitors and friends/families. I'm in the 750 MC and its whole commercial race promotion depends on entry fees not spectators.
Having said al of that I will now contradict myself a bit and mention John Webb and the Rolling Stones. John Webb was the 1970's Brands Hatch promoter and an outstanding one. Once he got 50,000 people to a club meeting via radio promotion. Of course club attendances were higher then but fewer than 10,000 so he showed what good promotion can do.
Similarly the Stones turned live gig touring into a financial monster show and many acts, new and old, now derive most income form live, and very expensive, shows. Again good marketing a help live attendance despite so much cyber media supply.
So maybe the problem is that eh skilled promoters have vanished?