The 500 survived the IRL years, so it's not like some empty seats are a disaster. It's still enough of an event to attract 200-300k people.
I do believe auto racing is on a decline, and will not attract enough new fans to compensate for those it will lose due to an aging demographic. The world has changed, and driving a race car, even at 230mph, isn't considered as daring of a feat as it used to be. Many more sports/diversions to fracture the audience, and many more media outlets, with each grabbing a little piece of the pie, instead of a few big slices as before.
Horse racing may provide a good idea of where things are headed. Attendance is down at local tracks, and they are disappearing. Those who still maintain an interest can engage in OTB. Yet there are still the big events like the Triple Crown, that will attract the diehards, but merit just a trivial blurb of attention otherwise.
So, there will still be the 500s, the Daytonas, and Le Mans, but the series built around those marquee events will find it tough going.