This is already happening in areas that are losing population, mainly the Northeast and parts of the Rust Belt. Those areas used to be the most densely populated areas of the U.S., but in the last 20-30 years have had severe brain drain, as well as talent drain. Not even exaggerating, there are tons of towns/small cities where literally 2/3 or even 3/4 of the population is over 50. Most of the younger gens move elsewhere as soon as they reach adulthood, most millennials with kids move elsewhere for better jobs/schools. Hence, many smaller colleges are merging with one another, or simply closing outright.
This problem is almost entirely a Northeast/Rust Belt phenomenon, the South and both Coasts are still experiencing full college admissions, with some schools even expanding.
If there is a larger shift away from college in the future, that might change, but my anecdotal observation is that most college closures are still happening in places with aging/declining populations. I suspect this might also be a reason why some colleges are desperate for foreign students to the point of overlooking cheating among foreigners or other bad behavior.