I think you also have to consider that Nielsen has adjusted their method of collecting their data in the past few years. Note that viewership of everything is down including the NFL by 13% according to this article.
http://detroit.cbslocal.com/2016/09/20/are-athlete-national-anthem-protests-costing-nfl-viewership/
I think you also have to consider that people are cutting the cord in a big way and it is getting much more difficult to get an accurate reading on the number of people watching. How do they account for the number of people watching at a bar? Or watching using OTA antenna? or stealing the broadcast from an online stream?
I'm not saying that NASCAR hasn't lost fans, but I think the numbers aren't an exact science. I also think the economy has a lot to do with people going to the races. I think those fans are still watching, just not from the race track.
"which stick and ball team has reduced seating from 25 to 50% and lost half its at home audience?"
Have you been to a baseball game lately? Half the stadium is empty unless you live in boston, ny, or chicago. This is a direct result of the tv contracts that these teams have signed. The teams get multi-millions of dollars from a tv channel and then crank up ticket prices because they can and don't care about people in the stands because they have already made their money from the tv contract. NASCAR has done the same thing. The problem is, I think it affects NASCAR more than it does baseball teams just due to the economic status of the fans of each sport.
As for the chase, I'm a fan. It adds drama to races that otherwise would not be talked about. The playoffs of any sport put everything on the line. Nobody cares who won the regular season of the NFL last year, or baseball, or hockey. Lower seeded teams beat the favorites all the time, i.e. march madness. That's why the game is played and not just on paper. I think the chase is the least of NASCAR's problems and that it stems more from the paper mache cars and cookie cutter tracks. People enjoy watching drama and NASCAR has lost it's drama the past decade. I think they are making steps in the right direction (not the caution clock) but it takes a while to right the ship.