" I don't see how life here is that much better for illegals, at least initially. The ones here legally have/have had pretty decent jobs, and are moving up the economic ladder to become part of the vanishing middle class."
You have to be kidding. Life here is far better than anything they had back home. Plumbing just across the border can be hit and miss. Some don't have running water. And the water is polluted. Corruption and crime are rampant. You have to pay for medical care, so it is much easier to go to America and get the freebies. I would encourage you to take a trip to El Paso, TX for instance. Drive along the US interstate, and look across into Mexico. The differences are really evident.
I don't think they come here because of the plumbing. Unsafe water isn't so good. I mentioned corruption. Many of them seem to live in shanties with no screens or windows, just a hole. I just wonder if they know how much more everything costs here relative to the low wages they are willing to work for. Somehow they seem to manage. They keep coming. Something else is driving this besides just the "American dream."