The simple fact of the matter is, due to the physical nature of thee work, these types of jobs have at most a 5 year cycle. That's why in the 'olden' days (pre-illegals), people between 18-25 were to be found toiling away outside in the hot sun. (Also why you could suck down a six-pack at the end of the day without gaining any weight.)
Illegals are not supermen - they also get burnt out and can no longer work the same jobs as they age. They themselves are trapped because younger illegals coming over the border displace them in the next cycle. But since they have no other marketable skills, don't know the language, etc, they can't move up the value chain. Instead, they become a drag on the economy as they consume increased social services.
What we have no is very similar to Dickensian England, where absent child labor laws, business owners just cycled through each generation of youngsters. Take a look at the political rationale behind banning child labor - business owners were foisting their broken employees on the backs of society, just as they are doing today with illegals.
That was a time now called BO(Before Oprah).
Excellent post.