What are the consequences of the declining birth rate in the U.S.?

The US population overall is increasing, but not through the birth rate. Most of the increase is coming from increased immigration.

Is this something to worry about? Why? Do we want a declining population? Why?

If we have a declining population, will we see Detroits and Youngstowns all over the place? If the cities do all lose populations dramatically, and vacant homes pop up all over the place, is this a problem?