I don't think I ever said or suggested that the point of the immigration system is to create a pool of illegal immigrants working outside the protection of labor laws. It is a consequence that some people are taking advantage of, but I doubt there's a grand conspiracy pushing for this.
If the conclusion that the purpose of our immigration system is to create an underclass for circumventing labor laws is something that's been obvious to most people for some time, I'm curious for more reading on the matter. Especially since, as you say, most knowledgeable people have reached this conclusion. If this is the case, it's presumably stated clearly and unambiguously in multiple media outlets.
While allowing anyone who wants to come in legally would fix the one problem of employers taking advantage of an underclass, it would still have significant downsides.
1) It is currently very unpopular. The majority of Democrats do not want an increase of any kind in legal immigration. This push would likely have a major backlash.
2) It is going to require significant infrastructure changes, given the massive increase in population.
3) It will change the composition of the country in unpredictable ways. If we allow a lot of immigrants full citizenship very quickly (say 50,000,000 adults in three years), that might not be conducive to assimilation, and can have unpredictable policy implications when they exercise their voting power, especially if they come from countries that aren't
WEIRD (Western, Educated, Industrialized, Rich and Democratic.) Some of the strongest claims for Asylum with the caravan-seekers came from LGBT Migrants fleeing persecution at home (El Salvador, Honduras, and Guatemala.) What happens if tens of millions of others take those attitudes to the US and form new voting blocs?
4) If these new immigrants receive any former of government support, that's going to be expensive.