On this forum EP On Road is definitely the king, but there are other places where TC's are rarely discussed... Then there's the newbie site run by a certain magazine with a lot of 10 year olds who want the fastest nitro Mommy can buy and break it in the first 5 minutes...
As for what people are racing and what's declining depends solely upon where you live. You need to look at the turnout at your local tracks. 12th scale seems to be getting more people lately, probably because of the fact they aren't as complicated and work well on tight tracks.
Off road has always gone back and forth between Buggies and Trucks. Now you have Truggies and Monster Trucks which have added more classes but to me Monster Trucks are for sand pits not racing....
A stadium truck is never a bad purchase, even if you can't race it you can always run it almost anywhere.
I'm in this hobby for the long haul, I don't really care what's popular now. I got my first RC in the 70's and I hope to still be doing this when I'M in my 70's....