First, most illegals are not coming here for work, they think this is their country.
They are not going to shut out other Mexicans.
Second, the fence is built with heavy machenry, tech support, and a sophsticated operation. This is not a simple fence.
We have had troops at the border but to do this job would take tens of thousands.
You seem like an intelligent person. I am, too. As a matter of fact, I held a general contractors license for a while. I have also been involved with concrete in the construction of several large bridges and a nuclear power plant, in NC.
While what you say has a measure of truth, in reality, IMHO, the whole thing could be completed within a year. You must not have waved any greenbacks in the face of a mexican laborer! You must underestimate the abilities READILY available here in this country. All it takes is a good leader... with a good imagination!