Synopsis of comments and my 2 cents…
1. Index classes are participant classes, not spectator classes: This is absolutely correct. Having run index classes I can tell you it’s a lot of fun, but that isn’t really evident to spectators. Usually the spectators of these classes are crew, family, and friends. At Big Show events, it’s hot dog and a soda time for most fans. Plus being all-run classes your weekend doesn’t end after a few qualifying shots if you don’t make the cut. That's attractive to racers, but spectators probably don't care.
2. NHRA enjoys the back gate money: You bet they do! Huge car counts helps pay operating costs. Don’t expect to see these classes going away any time soon.
3. Lowering the index a second for each class would help spectator appeal: Maybe, maybe not. One idea I’ve thrown out before is to enforce a speed limit to kind of force cars into the class they should be in. I say this because I’ve seen 7-second cars running 10.90 class and crossing the stripe at well over 170 mph - but it was on the throttle stop for over 3 seconds, which would look odd to spectators. The poor guy with a 130 mph car never saw him coming (which is the strategy).
4. TD & TS rock: Agreed! Yeah it’s dial-in racing, but at those speeds you're too awed by the performance to notice.