I'm willing to bet that you are not in the fencing industry or a security expert that would make you informed on the possibilities of "success" of a border wall and its' design elements one way or the other - excluding, of course, what we all got from the repeated, multiple times per day mantra from the D's, the reports from msm, and the parade of articles from WaPo, NYT, The Atlantic, etc.
No one ever said that a wall at the southern border would be 100% effective. It doesn't have to be. Conceptually, either walls and fencing work or they don't. The rest is just application challenges. The CBP reported that they had over 239,000 "encounters" just during May. That's the ones they know about so imagine the real numbers. If a wall helped just cut that in half due to your "massive system of tunnels, rivers, etc.", how is that not a win? It's about telling the massive "wall" of immigrants that we aren't just completely open with Joe holding up the neon "Vacancy" sign. As to your original premise, there are some who disagree with you:
I fully admit that walls are more effective when shorter and less to be maintained or defeated through other means. But, that doesn't mean that they don't work or that they are not part of an overall strategy that should be employed. And therein is the issue: walls do work to some extent in almost every situation, but the question is whether keeping people out is actually the goal.