doesn’t get sick

Oh machines get sick and then the real costs appear. (Am an electrician, I play doctor on automated machines, sometimes a human is cheaper when the whole plant stops for a $200 part that is not in stock)

however little they’re paid, they’ll always be orders of magnitude more expensive than a machine.

No... they're not. The cost is going to depend on a lot of things - which includes the profiteering of the capitalists that's going to own the AI.

Point is, it's not the costs that is the problem - that's just the excuse capitalists use. They can externalize costs, or get the poor to subsisize their costs - what the capitalist class has always done. The point is class warfare - they hold the people who make them rich in contempt and they always have, and this is the case irregardless of whether the workers in question...

doesn’t complain, doesn’t rest, doesn’t get sick, doesn’t organize walkouts

That's the whole point of hierarchies - the people at the top is incentivized to, and has the power to, kick down no matter how accommodating the people at the bottom is to their parasitic needs.