Part time jobs are available and they are often taken by people who work multiple jobs.

Sure, that is another avenue of economic erosion but I didn't focus on it because the "exempt" dynamic would seem to be the stronger dynamic. But if we actually fixed the flaws I listed, we would likely have to plug that hole of multiple jobs not counting towards the full time limit, especially if full time employment were defined as 15 hours per week (leaving plenty of time to stuff in a second or even third job in place of "visiting grandma")

But in general here we're talking about the common case. There's no real way to stop an entrepreneur from dumping 80 hours per week into their own profitable business. The important part is that such things not be common, to prevent the result of many things just being bid up in price and making that amount of time spent de facto mandatory.