Ok, you're pragmatic about it. I agree that on paper, it looks "cost-effective" (actually, I haven't seen an estimate of the amount "saved" by the wall - the cost of immigrants won't disappear. It will go down by a rate of number of illegals that leave vs. the number that make it in, which I haven't seen estimated.
This begs the question: will the wall really be effective? The first thing that will happen is that those that are determined will start looking for weak spots, devising ladders, digging tunnels, and so on. And, as you mentioned, people will start looking for sea access, and via Canada.
Note that we are having, in parallel, a discussion that is ongoing with true experts in the field, with much lower quality here in terms of information. This is one of the main points of my project: create one place where the experts can put it all in and sort it out once for the benefit of everyone.
A wall is just a wall, just like a police force is just a police force, and gun is just a gun. The symbolism lies not in what it is, but how you wield it. A police force says, "we believe in the rule of law". Unless those police are encouraged to beat political dissidents, or people of color, or... In other words, a wall, built slowly, as part of a national security program, coupled with DACA and programs to nationalize or give working visas to existing immigrants, and a rebalancing of the role of immigrants in our community and society shows that the U.S. recognizes the contribution of immigrants to our society, and that we want to better control our borders. A wall built by a white billionaire supported by the KKK that has accused the country on the other side of sending its worst, its criminals and rapists, and threatening to make them pay for it is a very different message. It isn't just idealists vs. pragmatists.
Right, the so-called philosopher king effect. Of course that's the most effective. Why isn't that the preferred form of government? Because it's a matter of luck. As you've pointed out here, the odds of it coming out well aren't so good ("the only in the history of the world"... actually, there are many more cases, but in general, it's best not to take your chances). Another, possibly the BIGGEST benefit of a democracy, is the peaceful exchange of power. How many people have died because the current leader passed on and factions started vying for power? Too many.
My idea is something more radical that hasn't been tried yet, because it requires technology. Here's a big IF, but let's say my debate site gets built, becomes as universal as Wikipedia, and becomes the central place for political debate. How the site works is also too long for me to describe here, but assume that the average voter can look up in 30s the major pros and cons of anything, and know that those were fully vetted by all the experts. I now, big assumptions, but stick with me.
Now, say that we put proposals up for direct vote with citizens. Not just big once-in-every-four-years votes, but "should we designate this building a historic marker for preservation?", "should we spend more money to protect the endangered koos-koos bird of the Mortinakinaye Swamp", "should the TPP include a clause to allow businesses to sue foreign nations for damages" and "should the FCC permit internet access providers to charge content providers for higher bandwidth rates?"
Too many issues to vote on, right? And, there's no one designated day to vote. Voting is an ongoing thing. So how would this possibly work? The short answer: most people would not vote on the majority of things. Many economists have said that one of the problems with the one-person-one-vote system is that there's no way to measure HOW MUCH someone wants something. A casual voter (or one forced to vote by mandatory vote) gets as much power as someone that has dedicated their entire life to the cause being voted. In my proposal, that would still be true. BUT THERE WOULD BE NO CASUAL VOTERS. There are so many votes to be cast that people will naturally gravitate to the issues that most impact them, and those for which they are experts on the matter. Would that work? I don't know - it's just a thought experiment in my head right now, but it would be interesting to test out.
The other advantages with this type of system is that there's no reason to "shock and awe" with changes to administration like Trump is doing, and like Obama did. Your first act as new president should not be to destroy everything the previous one did. Look at Obamacare: wouldn't it be better to examine its problems, and find small adjustments to make (and measure)? In a pragmatic sense, yes, but politically, Trump must be seen as destroying it. Tsk tsk tsk.