If genuine labor shortages exist, then laws should be modified to address as they occur and handled in a process that matches demand legally; the fact that it continues unaddressed for so long indicates that their is no desire to find legal remedy because it would mean that employers would have to treat these workers and pay these workers like American citizens which is what they are trying so hard to do. Once this group of illegals are legalized, employers will have no need for them and start hiring the illegal ones again.
I think the point is to get rid of the backlog of illegals that have gotten in over the last 20 years or so. From what I understand from LE people, the border is now tighter than it ever has been, and it's no way near as easy as it used to be to slip across. So I think the problem is to clear up the huge number of people who are already here - get rid of the criminals (who Mexico, naturally, does not want to take back), get genuine IDs for the ones who are employed and not criminals, and set some time limits (such as 3 years, for example).
But I think once that large group was dealt with, any new illegal arrivals would be nowhere near enough to fill those jobs. Probably what would happen in the case of a lot of truly low-wage agricultural jobs is that they would be mechanized, in any case.
Yup, the NEW illegals, and there will be millions more of them because we "solved" the problem by amnesty, remember? The border is STILL not closed.