That's a good question, Dominic!
What the middle have done has shifted over the centuries: from outdoor physical labor of farming, to dextrous craftwork, to repetitive factory work, to light mental labor of office work, to light indoor labor of personal service. Yet it's less about a true shift than it is about an expansion of roles -- there's still artisan farmers that live by physical outdoor labor and or workers in physically-demanding construction work. Yet there seems to be an increasing amount of service labor (restaurants, retail, e-commerce fulfillment, sales, entertainment, travel/leisure, etc.)
I also wonder about UBI (Universal Basic Income) as an alternative to social safety net systems and minimum wage laws. Current social welfare systems seem horribly regressive -- the loss of social unemployment benefits caused by getting a job seem like a near 100% marginal tax rate. And some people's labor is simply not worth minimum wage although it might be worth something and lead to an accumulation of skill and experience that increases the value of their labor over time. UBI solves both those issues. But..........., of course, it's not clear whether the higher-wealth people would tolerate the increased taxes or emigrate to a less tax-intensive country.