Once upon a time, we welcomed people from enemy states. They were called "defectors." Now, apparently, the rule is that everyone from an enemy country is an enemy, with the exception of the leaders of those countries who butter up Trump and become his "dear friends."
There has been no "fallout" and there never will be, because a) everyone was vetted and b) they are migrants, not foreign agents. Again, the easiest way for terrorists to come into the country is not crossing the Southern border. How did the 9/11 bombers get in? Visas. If there were to be fallout, we would have seen it already -- it's not as if the asylum surge was new (lots of border crossing in 2016-2019).
Bottom line: there are very few foreigners who want to come into the US to be terrorists. The risk/reward just isn't remotely worth it. It requires money, which isn't necessarily available without bin Laden, and the terrorism groups have decided that bin Laden's strategy of going after the "far enemy" was ultimately stupid. While it did wreck the US, it did nothing good for the Islamist cause. ISIL was more effective by far, and thus the terrorist groups remain focused on the near enemy.
But y'all are the people who are also terrified of illegal voting, even though it doesn't occur for the same reasons. It's not remotely worth it.
Maybe it's a bad idea for the country to sell out all its principles, erode our liberties and civil rights, and elect nincompoops based on imagined threats that have no basis in reality. Why y'all so scared of everything? Living in fear, my god.