The number of Detroit automakers has been contracting since World War II (the movie Cars reintroduced us to the Hudson Hornet, which became part of AMC). Unfortunately, it's probably time for another contraction.
Toyota, Nissan and Honda are a pretty big three in car making now, with plenty of capacity in their U.S. plants. With the baby boomers starting to retire, they will not be as hard on cars and need to replace them as quickly. The current car building capacity is probably way too much for future trends, no matter what happens with the economy.