What if you want is to keep a human alive. How can that cost less than keeping a human alive?

The assertion that we are refuting is this: "human wants are infinite, and therefore, we always find new things for people to do". The context here is specifically about employment, human labor, and the spectre of joblessness. What you're describing is not a labor market, it's charity. And indeed, one peaceful, idyllic solution to mass unemployment that gets trotted out is something like UBI, where you pay people to simply keep them alive, without expecting anything in return. But that's not at all what's being discussed here; instead, what the OP is asserting is the usual yarn that technological advances will not decrease human employment, but at a certain point this simply stops being the case, and that point will be reached if or when the price of artificial labor falls below a critical level. In short: at the limit of technological advancement, you can either prioritize a market economy, or you can prioritize keeping people alive; you cannot have both.

Great question that will unfortunately be ignored by GP, as it happens usually.

I don't think anyone pushing AI really cares about keeping humans alive

AI is a fundamentally antisocial anti-human technology