[rmlew]

Low birthrates are a problem, not the only problem. I'd settle for Americans actually having 2.2 children per woman.
Japan has far worse fertility rates than us, but they are not replacing their population with foreigners. A Japan with 1/3 less population will still be Japan.

[Monorprise]

The birth rate of non-Hispanic whites has been even lower than the national average.

You may be right on the immigration issue, if we could just shrink without replacing the population with un-assimilated foreigner we would survive. (Although with serous economic, and defense problems.)

This remark of course brings us to the next major problem. Thanks to liberal ideals of multiculturalism, we are not assimilating immigrants to productive american culture as we need to be. Instead them immigrants are being encouraged to hold onto their old country culturally rather than embrace & merge with their new country.