From the viewpoint of being seriously involved in motorsports for 45 years, NASCAR has been in steady decline for 40 years, and rapid decline for 25. The Brian France era began the rapid decline, and it has not abated.
What made NASCAR was the use of an actual two door sedan, with production based body panels and frame, and a production based engine. When Bill France Jr. allowed the smaller sports coupe, and then the front wheel drive cars that devolved into jelly beans, the decline began. Then they allowed engines designed purely for racing. After that, it became nothing but tricks and gimmicks. There was also the foolish pursuit of "new markets", where they abandoned the hard core fans, and the tracks they loved, in order to try to bring in "new fans" and those cookie cutter 1-1/2 mile tracks. They took racing away from people who loved it, closed their tracks, and told them "too bad". The "new markets" failed, and the racing on the "cookie cutter" tracks sucked.