The problem with Trump’s wall is that it's a solution without a problem.
He just wants to build a wall. He's always wanted to build a wall.
Trump thinks of himself as a guy who builds stuff (even though he doesn't actually, of course, build stuff...doesn't do the physical work, doesn't design the project, doesn't even do the actual development anymore). But he has an idea of himself as a guy who builds things.
He doesn't like black or brown people (but black people are worse). He's never liked black or brown people — he's an old-style German Aryans-are-better eugenicist.
His concept of himself is of a guy with “good [ie white] genes”.
The key to Trump is that he's not very smart, and he's ideologically rigid, and he's a very shallow thinker (all qualities of narcissists, who can't think too deeply about anything or they are confronted with the dark hole at the center of their being).
So when he was running his handlers put the two bedrock qualities of Trump — builder + racist — ideas together and crafted that into a kind of symbol, The Wall, that they needed to gin up the folks they needed to gin up. Some of those folks were racists, some were economically disenfranchised, all were feeling powerless in an age where you have to be a corporate lobbyist to have any say in what your government does anymore.
so...The Wall. It worked because it was a symbol. Symbols work with people, and it worked with Trump, too, because it fed into two qualities that he actually really believes in about himself, again, he builds things, and the USA is better when darker skinned people are in other countries. So Trump was especially convincing when he talked about the wall because he believed in it. He was less convincing about other stuff his varying staff of flunky enablers got him to say — stuff about Medicare, for example. Because he didn't really care about it, didn't understand it. But “build that wall”& to a lesser extent “drain the swamp”...he understood walls, & he understood swamps (or at least wetlands).
But the problem with symbollic solutions is that they don't solve literal problems.
The Wall will not solve any of the messy problems of immigration. There is no actual Swamp to drain. But Trump has never been able to understand this.
He wants to build a wall. He is looking for a problem that a wall would solve.
...but the only problem a (let's face it) $90 billion wall solves is the problem of “all this extra money is getting in my way when I try to walk down the hall, it's too damp with swamp water to combust, how can I possibly get rid of it?”