We could eventually replace (parts of) the workforce if we were happy with the current standards of living (and prevented a small group from hoarding all the wealth). If we were satisfied with a 1850s standard of living, only a small portion of the modern workforce would need to work. What automation enables is greater productivity with the same amount of human effort/time. But because we seek to improve out living standards, we need to keep the same portion of people employed and productive. So I say, once we bring everyone to a decent standard of life, we can just stop and gradually liberate people from the shackles of work until society runs on a bunch of robots and a small core of maintainers.