Track is dying everywhere, not just the US. Sports participation in general is dying out from a combination of busy parents/people, cotton wool parenting, technology and costs. I've been involved in a number of sports between myself and my 2 children, and comparing to maybe 20 years ago, across the board numbers are down for everything.
Multi use facilities are definitely the way to go. We have a lot of tracks built in Australia of the flat track type around a cricket ground. In general, they are surviving, but only just. Lots of purpose built velodromes are gone here in Australia too, mostly due to lack of use. They stop being used, then not maintained and then just become white elephants, too expensive to even demolish. But compared to the US, we still have a lot of velodromes around. We are even getting a number of new velos built as part of multi discipline bike park style facilities.