I agree with this more than anything. I think this would help them more than they think. I imagine their fear here is a loss of TV revenue for the networks as they would be losing quite a bit of commercial time by chopping 100 miles off the race/broadcast length. However, I think the old supply vs demand issue would come into play here, maybe they could charge a bit more for ad time with a reduced supply.
In the past, when I became a fan of the sport back in my late teens/early 20's I had no issues with race length. I had the luxury of having 4 hours to spend. Now that I'm in my mid 30's, I just don't have the time, or the patience to devote to a race that long when the majority of it seems just riding around. Sure, that's changed a bit with the stage racing, but all the stages have done is lengthen times even further it seems.
I agree though, drop 100 miles from the majority of the 500 mile races and make them 400. I think you've got to find a way to drop at least an hour off each event from a TV standpoint. Doing this, while keeping the crown jewels longer makes those events standout, and make them "special" My crown jewel list though would be something like this:
Daytona 500 - for obvious reasons
Coke 600 - Again, for obvious reasons of the tradition, and endurance.
Southern 500 (Only at Darlington) - For the obvious reasons of tradition.
Brickyard 400 - I know this race struggles, but it's Indy. Anything less than 400 here doesn't seem right. They just need to improve the racing.
I may be inclined to include the "regular season finale" in the list of keeping the longer race distance, along with the season finale at homestead.
That gives you a nice list of premier events based on tradition, and based on competitive situations/points in the season where you may benefit from a longer race.
Ultimately, there's no secret people's interest is declining because it's too long, at home, and at the track. Common complaint I hear a lot is weather. Especially in the dog days of summer, sitting at the track in heat and humidity for that long can be a struggle. I remember being at the June race in Michigan about 6-7 years ago. It was 95 degrees that day, and basically 100% humidity. A brief shower came through before the race start which ramped up the feeling of the humidity even more. The track dried almost instantly because it was so hot. 4 Hours, on a day like that, on a follow the leader type 400 mile race is just... insufferable to a lot of people nowadays.
I think they need to look at their ticket prices too. That's likely another key factor in keeping people away from the tracks on race days. They need to do a better job of getting people to the tracks. Reduced pricing helps that. Once a majority of people go, especially for the first time I think they get "hooked". It's pretty impressive to experience live, but that being said, unless you're completely into it, the desire to return year after year dissipates rather quickly for most people I bet. Reduce ticket prices, improve seating, improve shading in the hotter environments, improve traffic flows post race, etc. etc. There's a lot of little things that tracks didn't stay on top of well enough as the sport boomed 15-20 years ago. It's gotten better, but there's a lot more they could do though, but I get it, it's tough when most of those facilities only generate significant revenues 2 weekends a year when Nascar roles into down, aside from the few markets that attract Indycar events as well, although the revenue of course isn't as high.
TLDR: Reduce race lengths except for key events, reduce ticket prices, improve track experience.