It's gonna sound weird but the cars, tracks and drivers are just all too good now. Yes, in most cases, I find NASCAR boring too now. 4 hours of droning in a circle with a tiny bit of excitement in the last 20 minutes. You know what I enjoy more now? NHRA & Le Mans style sports car racing. Drag racing is great because every 5 seconds there is a new winner, and a guy from Nowhere USA can build a car in his garage & show up & compete against the world champion on live TV in front of a full house. True, it's rare that they win, but it does happen, more than you'd think. Endurance racing just has more variety so that's why I enjoy it.
My advice for NASCAR? Yes, take much of the aero out of the cars and try a few more tracks like the Charlotte roval. Basically it would throw a monkey wrench into the long and deep crew chief's playbook & introduce some unpredictability.