I chalk this up to one of the big negatives of the internet. I've been around the sport my whole life, I've seen tracks come and go, happens all the time. Now that everyone is connected, we get overwhelmed with all this negative news of tracks closing. Then we immediately think the sport is declining.
Well, I'm looking at real numbers based off actual results files from every AMA promoter in the country, those numbers show a positive trend upward in participation... at least at the one thousand Motocross events we sanction a year. I was talking with a representative of a OEM, bike sales are increasing. Now these numbers aren't where they were in the mid-90's, but no industry/economy is what is was in the 90's (for the except of electronic/handheld devices).
Keep in mind, building and running a race/practice track, if you're trying to make money, it is a business decision. People make bad business decisions like building a MX tracks away from any population, investing money in their facility inaccurately, bad customer service, not willing to adapt with the changing market, unwilling to partner and work with others. Your right, great tracks go out of business, but good business owners don't.
Please take one thing away from my comment... participation in AMA motocross events is up and it's rising...