inconsistent The only places at which they use restrictor plates are Daytona, Taladega and maybe one other track. As to attendance NASCAR is down but compared do other US race series they are miles ahead. At IMSA, Indy Car etc. most of the spectators show up looking like empty seats. the only exception is the INDY 500.
Internationally F1 does O.K. in some places but in others like Germany not so much.
I think the overall problem is younger people spending all day every day playing with their cell phones or Ipads. How many people do you know who spend 5 minutes a year working on their cars? An even smaller group would be people who even know how to do basic maintenance or make a simple change to the factory provided hardware and/or software. The class of people who know/knew how to perform these tasks were the backbone of the continuing racing fan base. They are disappearing and replacing them is a continuing problem for all racing.
The problem with the racing itself is way to much aero design. Once that becomes the overriding design criteria then all the cars tend to look/perform the same. It also is the primary cause of all the wrecks. When your cars ability to stick in the corners is primarily aero and not mechanical grip, close racing which disrupts the airflow, results in a lot of wrecks.
Watch a Formula 1 race. It is almost impossible to pass anyone since getting close causes your down force to disappear. The only way they get any passing is to have a drag reduction element in the rear wings which you can open if you are within one second of the car in front of you in some straight sections of the track. In INDY car they have a limited amount of push to pass hits to the engine which adds a bit of horsepower for a short time.
It's my opinion getting rid of the tacked on aero pieces would result in a lot better racing. The lap times would slow down but so what. The show would be a lot better with less wrecks, especially when no one is really doing anything but riding around.