I see your point, and I see your concern. However, I don't think that concern alone is a stopping factor for implementing such a decision. That would mean that you cannot make progress in many bad situations, when people have begun to rely on them. As an example, I think Obama was right to pull out of Iraq, but he was criticized for increasing the instability there and the return of terrorist groups to it. The Republican support from the Rust belt is a similar example(moving from cheap to intellectual labor has been great for the USA's overall GDP, but has destroyed the livelihood of blue-collar workers).

That was the heart of my point - with a limited budget of resources, you simply cannot help anyone. At that point, you are faced with some philosophical questions - should I be maximizing some global 'goodness' factor, or should I be working for the local group that I am supposed to represent? In the end, the leaders of democratic countries are elected to represent the will of their voters, not to better mankind(unless that is what the voters want!).

If Trump and his supporters believe that a wall will improve the situation for the US and American citizens, then I don't think arguments about the welfare of others(whom they might even view antagonistically) will sway them.

I found different arguments much more persuasive - arguments that removing illegal immigrants would not bring jobs back to American citizens, because they were a net economic positive and creating demand as well as supply.

However, these arguments seemed to be drowned out by the loud, resonating narrative on how evil and racist the supporters of Trump's policy were, which I can only imagine hardened their resolve to get it through. An especially easy angle that Trump could pursue was the disregard of rule of law by illegal immigrant supporters, which I feel could have received a lot more attention and understanding from Trump's opposition.

Fair points all around. Thanks!