If Jamie is happier working a job, then fine. But was he born wanting to get a job? How much came from him and how much came from his parents placing unrealistic expectations on him?
Do you honestly believe that low-skilled workers are being kept from doing more mentally demanding work because they can't find anyone to take over their low-skilled jobs?
Your strawman argument is irrelevant. Even assuming that the supply of jobs is fixed, which it isn't (lump of labor and all that), here are the choices:
a) overqualified person, here meaning somebody with an IQ of 100, works at a shit job and hates it; and person with Down's sits in basement watching YouTube all day and hates it, while society subsidizes his existence
b) person with Down's does a monotonous job and is happy with it; and neurotypical person is unemployed and can choose how he spends his days while society subsidizes his existence
I'd go for B, especially given that it gives the other person the chance to do something else useful. They may or may not actually do it, mind you, but at least there's a chance, while there really isn't in the other scenario.
Perhaps the GP didn't get the causality quite right, but we are entering some kind of a dystopian twilight zone with regard to employment. Not too long ago, there was the story about a Walmart opening up in the D.C. area, and that one single store received __25,000__ job applications. That's a lower acceptance rate than Harvard, getting a job at Walmart is literally more cut-throat than getting into Harvard.