I am sure it is true that some Americans won't do dirty jobs- but many will and have had their jobs taken away and given to illegals for the past 30+years that I know of. That mantra started in the agriculture industry when they wanted to hire illegals because they were cheaper and would put up with unsafe and otherwise terrible working and living conditions that citizens wouldn't. My family worked on ranches and farms for years and beginning in the 1970s many ranchers and farmers made a decision to hire illegals instead- not because they couldn't get Americans. Plenty of Americans still work on ranches and farms and in packing houses- etc- what is unfair is those ranchers and farmers that won't hire illegals have to compete with those that do, so they make less profit. It is costing businesses money to follow the law and do the right thing. Illegals then took over the construction jobs- not because there were not Americans that wanted the jobs- because again it is cheaper and less hassle to hire illegals. Same for the hotel, restaurant and other businesses. Americans were working those jobs, until illegals were allowed to pour in and take them.
I want you to think about a couple of things- Americans do jobs like morgue work- septic system work- rodent removal- many many horrible jobs that are nearly exclusive to Americans, not illegals. There are Americans doing all the same jobs illegals do. Illegals are less than 10% of the work force so there has to be a lot of Americans doing "dirty" jobs. Don't make the mistake of thinking like the politicians do that just because you or people you know won't do the work no American will. I have done nearly all of the jobs that are considered to lowly for Americans and have friends still working those type of jobs.
Bottom line is money- meat packing house work, other work related to agriculture, construction, and service jobs were all decent paying jobs at one time. Men could get in those jobs and support their family and work their way into a better job or even a business using the bottom tier jobs as a stepping stone. As soon as illegals take jobs in an industry, all the wages are kept low, benefits scarce and working conditions deteriote to the point where Americans may not be able to take the job. I was shocked to learn during the housing boom that many construction jobs paid the same as when I worked for a construction company doing payroll in the early 1980s. How many jobs can you think of that are paying 1980 wages? It really is about money.
Americans cannot compete with illegals for low wages- the fact is illegals cannot get by on the low wages either, which is why they have over run our social services to the point of bankruptcy of state government using the medical and other social services.
The only jobs I can think of that we might need illegals for is harvest work on farms. The reason for that is the job is temporary so it is hard to get people to do that work. Not because it is work people are too good for, but because it is a temporary job and it is hard for anyone to count on that for a living. There are programs for farmers to get temporary workers for harvest, but many choose to not participate because there is red tape involved and again it is cheaper for them to hire illegal workers so many do. Not all farmers hire illegals so again it is not fair to those that follow the law to compete with those who don't.
I cringe when I hear "Jobs Americans won't do" or similiar statements- not factually true and is simply an excuse for some business people to break the law to improve their profits, and gives politicians an excuse to condone and support breaking the law or supporting amnesty.
Name one job that Americans aren't doing right now that illegals are. Just one.
You are correct that illegals are taking jobs from Americans and lowering wages for blue collar Americans.
Pre-1860, many poor southern whites moved north to the free states so that they would not have to compete with slave labor. Today poor Americans are competing with illegal labor.