Sure I can justify this. You present 10,000,000 employed people working jobs which could, conceivably, be automated. Do you seriously think that given technology currently applicable, their employers would not replace those people with machines in a heartbeat if the latter were cheaper? Sure, you can hypothesize about how that should be the case, but you would overlook a multitude of realities that indeed render the human worker superior to the automated alternatives.

I dislike human-clerk checkouts at stores. Nothing against people per se, but as a high-tech introvert I'd rather do it myself - exactly as you suggest. Sure, the technology is there; heck, Walmart even has a phone-based self-scanner so you can do 95% of checkout before you even get to the self-serve register for final payment. The technology sounds great on paper and in rhetoric, but in reality it sucks (despite my valiant concerted efforts to be a automation-supporting customer). Standard self-checkout chokes on the bottle of wine ("Human clerk, is customer over 21?" Uh, yeah, I'm greying with two kids in tow, of course I'm of age). Phone-based on-the-go checkout is time consuming (stop, turn on phone (again), tap Scan, point camera, wait for slow auto-focus, wait for crappy in-store wireless connection to function, get to payment station, get selected for another 15-minute "you've been selected for a compliance check" which confuses human staff every time). Never mind shoplifting, mis-scans, and a host of other problems. Those ten million "technology replaceable staff" are still employed because they're better at the work than technology, and relieving them of their duties does NOT make their freed-up wages available for confiscation & redistribution as "living wages" to the now-unemployed former workers (it's going to technology costs and high-tech maintenance staff). That's 10M people "contributing to maintenance of society".

As for manufacturing wants and increasing money spent vs. consumers making rational choices? I'm watching a startup put serious money into marketing staff; the product is [r]evolutionary and WILL "advance society & technology", but isn't going anywhere without convincing a lot of people to buy it, and the staff IS earning their significant wages by doing so. Oh sure, there's a lot of sales of superfluous goods out there, but even that helps fill out & support an infrastructure which gets vital goods to a broad clientele: Walmart isn't going to get five pounds of bread flour on a shelf for $1.89 without the delivery system greased with the profits from the "cheap crap" they're famous for; in comparison, that same sack of flour would cost about $5 at everything-is-perfect Whole Foods.

Indication that considering what is and isn't necessary in society could "liberate" half the population? Yeah: every society that tried it, like the Soviet Union (hint: they'd kill people for trying to leave).