Well, "decline" is a tricky term.
Hard to fill stands when every race is televised--and you get a better view of the race on tv (watching from the infield is cool and all, but you only get maybe a 15º wedge to see, so you are eyeballing a jumbotron or the tv in/next to the RV.
The next generation of drivers is pretty exciting. I'm really keen to see how Bowman does with his Pole at Daytona. Chase Elliot is more than due a win (and I was sorry he did not get one in the 24).
I'm also pretty certain some of us "olds" are going to be surprised to see some of these younger kids perfoem. The Cup cars are a deliberate handful to drive, you have to be committed to fitness and stamina to be in those cars.
Bbut, yeah, the influx of "cheap" asian-made diecast upon which "stickers" can be slapped is slaughtering the plastic model side of this.