Nobody forced the farmers to become reliant on migrant workers though. It was just a simple decision for them: cheap labor that's available today or expensive labor... They mostly went with cheap, and now they don't have anyone available. Tough luck I guess, but it always was just a question of time until this happened.
In America, it used to be common for young men in high school and college to do construction work during the summer. Now, it's mostly immigrants.
When I was still a military wife, they changed the fitness test to be less stringent for younger members because they were having so much trouble getting qualified recruits. It really pissed my husband off because they didn't make his test any easier. He was in his thirties and was told "You've been working out for years. You are in the best shape of your life."
Developed countries are turning into nations of couch potatoes who turn their noses up at hard physical labor and probably couldn't do it if they wanted to. Or could but it would be extremely hard on them and there would be additional personal costs akin to training for a marathon.