Trump can't figure out a key way Congress would undermine his "wall"
One key way "Trump Wall" would fail is if Congress neglects upkeep. Trump, as always, can't think ahead and plan for what Congress - almost all of which supports loose borders - would do.
Speaking to Joe Kernen of CNBC on his trip to Davos, Switzerland, Trump said among other things this (link):
But we need a strong border. And to do that you need a wall. And [the Democrats] did agree to a $25 billion wall, of which I'll have a lot of money left over. Because I don't need $25 billion to build a wall. We'll build a great - that's what I do. We'll build a great wall and we'll have a lot of money left over, and we'll spend it on other things. But we need great security.
The $25 billion will supposedly be in the form of a trust fund, which presumably means Congress couldn't withhold funding after agreeing to it and presumably means it would all have to be spent on the "wall".
However, "walls" - which in Trump's case is now walls, fencing, and bollards here and there - will have to be maintained. Fencing especially can be easily torn down, but even concrete structures can develop cracks, be damaged and breached, and so on. Trump will have built the "wall", but just building it is only the beginning.
Trump hasn't thought far enough ahead and doesn't realize how supportive Congress is of loose borders to have a plan to keep the "wall" in place. The Democrats - assisted by many Republicans - might even demand future amnesties or to weaken current amnesties in exchange for funding upkeep for whatever of Trump's "wall" that gets built.
If you're a Trump supporter or want his "wall", ask yourself why Trump wasn't capable of figuring out the second phase: keeping the "wall" in place.