What you seem to be advocating is interjecting large numbers of non-western, non-European peoples into the United States. Is that what you are saying? If so, I think that is a vastly bigger problem than cutting off all immigration. It should be obvious why. If you change America's demographic component, then it will change forever. It is never coming back. That is, what makes America America will be gone. I don't think that an America that looks like the UN is the America that you would like to live in. One in which multiple languages are spoking, different cultures abound, outright confusion, an oppressive centralized government to keep the bickering groups at bay. Furthermore, look at social identity politics. Imagine how much worse it will get.
The other option is to stop all immigration, of which America will see a slight drop in newborns. However, a 1.9-2.0 birthrate is not the catastrophe that Europe is dealing with. All that needs to be done is for abortion to be outlawed, as well as taxes lowered. With more wealth and the inability to murder ones child, birth rates will rise. Furthermore, American culture will live on, socialism will be kept at bay, and a chaotic nightmare America will be averted.
No, the birth rate is NOT going to go up, in the forseeable future; even should abortion be outlawed. Far too many people are either NOT getting married / delaying marriage / can't afford to have huge families / don't want to have many children, for that to occur. Your " wishes " aren't substanciated, at all, by facts and probability.
Of course not. However, let's take this as an example. West Germany dreamed of the day all of Germany would be re-united. When that day came even a welfare state like modern Germany was not prepared for the level of dependence on government that the newly freed east Germans demanded. They all came from the same cultural stock but their expectations, level of ambition and outlook is still a problem for the new Germany. It is not enough to talk about cultural purity without recognizing that cultural values are not genetic but learned values. Look at any town that has a history of being conservative and what happens when liberals migrate to it. However, I do have to ask, when did Mexico cease to be a "western" nation?