The effects of the "future generations" will be even more devastating if we let in groups of people who are entirely unlike ourselves, not just in race or ethnicity, but religion, culture, tradition, and society.
I would rather have a country with no immigration, like Japan, then one with massive immigration of 3rd world peoples, like the USA. Which one do you think is going to end up worse in the future?
Lastly, the US fertility rate is 1.9 births per woman.
Also,
1: We can continue to take in people from Europe
2: Read up on population momentum. We have a below sub-replacement fertility rate, but Population momentum could keep us positive for generations.
3: Immigration from 3rd world countries is not a solution to the low fertility rates ANYWAY. This just creates added conflict. You know how to get racism to stop? Separate the races.