People still illegally immigrate to the USA right now despite everything that is going on. The reason is also still the same and I see no reason why that should change in the next decades. I mean other than a civil war, but that is not going to happen even if Trump grabs power so...

The US has a lot of space and a lot of farmland. Most countries don’t. As ocean transportation of food (amongst other resources) becomes less reliable due to lack of security, people will be fleeing their failed or failing country to one that’s more stable.

You’re right to think that the US has shit policies for immigration now but I think that will change once population decline starts affecting how much food is stocked at grocery stores.

I anticipate things being a lot worse than they are even now over the next 15 years. Multiple wars, state-sanctioned sea piracy, and a failure in the complex supply chain that creates advanced chipsets used for things like servers/workstations, satellites, weather prediction, stock markets, etc. leading to a mini-collapse as a best-case scenario.