The alternative is paying people to do an unneeded job, and that's not sustainable.
Well unfortunately that's the proposed solution too. When you ask an AI optimist what their solution is for workers after their jobs are replaced by ai, a common answer is a universal basic income. But if you believe it's unsustainable to pay a person to do a job that could be done by a robot (which for the record isn't really accurate, as we've been sustaining that), then it probably isn't sustainable to pay that same person for doing nothing...
So we're left with the same problem, what do we do about the workers?