With NASCAR, you just don't see that kind of variance. It's been going down steadily for a while now and doesn't seem poised to stop anytime soon.
I had started a post with a similar theme. NASCAR loyalists make these isolated comparisons to try to equate NASCAR's position to "all sports". This ignores the larger context you cite. All NBA ratings have not been trending downward for 10 years. In fact, that league pretty much bottomed out a decade ago, and overall audiences are much larger than then. Ratings were up in 2015 and 2016. If the 2017 playoffs are down significantly, that's not a good sign for them. They signed a massive TV deal also that could turn out badly as well. But these two sports entities are not at all in the same position.
What professional teams sports like the NBA, MLB, and NHL have actually experienced in the last decade is:
1. lower national TV ratings during their regular season
2. higher local and regional TV ratings and improved attendance, generating much higher revenues on this level for the franchises
3. fluctuating but generally strong post-season ratings
There is little of that nuance and complexity with NASCAR's situation. NASCAR is down across the board for many years running, with the possible exception of the Homestead finale. They created a playoffs that covers nearly 1/3 of the season, and all they have accomplished in terms of television is increased attention on the final race. The rest of the entire farce meets with a collective yawn, including the elimination races.
All sports, like all entertainment, have become more fragmented and localized. Fans are interested in their local team only to a much greater degree. This doesn't really translate to NASCAR, especially because their focus for at least two decades has been shedding the perception that they are a regional sport to go broadly (some would say blandly) national. Only the NFL managed that, it's not attainable for any other sports property IMO.