The problem with the wall idea goes beyond the obvious that illegals will find other ways around it, and it also goes beyond the idea that the American tax payer will need to foot the bill instead of "Mexico".
The problem I see with not just the President pushing such an idea, but also the fact that so many of the public agree with the idea, is saddening. This world is already changing faster than what many realize. How many people even truly grasp what the advent of this technology we are using called the Internet has done for this world? At this point even if you are in some third world country under siege and missile are being fired every weekend only a few miles from your home, as long as you aren't killed yourself you can learn pretty much any high demand technical skill by just accessing your computer. You want a degree in Engineering? You can get it online, no need to put yourself at risk outside.
What this means is places that were once thought of as isolated are now having to deal with a population which is growing smarter and gaining more global information than ever before. When such people realize how other countries treat their citizens they are now more prone to rising up against their own governments. This thing called the Internet is already turning this planet into a global social society… and so people with so called "Nationalist" agendas are actually detrimental to the future of this planet in my opinion. People like Trump and those who agree with his ideas have some very obsolete thinking which does not interface with what is going to happen soon in the future. The entire point of a country is going to become irrelevant within a few hundred years or less. Everyone is mating with everyone (Especially where I live) so I can even foresee that within a thousand or so years there probably won't be too many shades anymore. We will likely move to one shade of color eventually and one huge society. The moment you understand this you realize how stupid the idea of something like a border wall is.