I ended up deleting my response because no good was going to come of it.
I'll post it here, because I worked so hard to summarize why Trump's anti-foreigner attitude is nonsense.
Terrorism in the first world is a trivial threat when you actually measure the costs in terms of loss of human lives, human suffering, and money lost. The wrong way to measure it is by how bad you feel when we think about it. Effort to deflect terrorism should be in proportion to the actual costs of terrorism.
What keeps terrorism in check is self-preservation. We live on a gigantic rock covered in people. Anyone can get onto the rock and kills lots of people if they wanted to. There is nothing the government can do to prevent that. It doesn't happen very often because they almost always end up dying afterward. Self-preservation can be undermined with ideology, but it takes a lot, especially when the rubber meets the road.
Immigrants don't need to produce more than average to benefit the economy. They only have to produce more than they consume. When they have access to machines in the U.S. they almost always do.
Immigrants are not all, mostly, and probably not even more likely to be criminals. The last one might be true, but there's enough data that says otherwise to believe that it's false.
There is no trade war with China. That's not how trade works. If they were sending us a bunch of cheap stuff and we weren't sending them anything back it would be good for us, not them. Economies are about outputs not work. Work and jobs aren't the kinds of things that we just run out of.