Agricultural technology already replaced most manual human jobs (from the era when most human jobs were agricultural). Humans found other things to do, and we found ways to use the surplus.
If it gets to the point where there isn't any unskilled labor left to do, we can always choose as a society to vastly expand the welfare state and divvy up at least part of the accumulated surplus to everyone. We have already moved in this direction a bit, and I expect to see more things along the lines of guaranteed minimum income in the future.
"vastly expand the welfare state and divvy up at least part of the accumulated surplus to everyone"
That's a technology we haven't had much luck with so far. We've had economies where "distribution" was related to direct wealth creation (make a sandwich and eat it myself), property & labour (you make a sandwich with my bread,we eat half each), thievery (gimme your sandwich). We've done sharing in small groups, that may have been the paleo-economy. We've doe bits of charity welfarism, redistribution and centralization but never really succeeded at making those work well at a large scale, especially for those supposed to be protected by it.
We disagree about how well redistribution can and has worked at scale. My ultra-compressed (lossy) take is "pretty inefficient, but somewhat effective improving the lives of the less-wealthy".
One of the most dramatic and promising examples of redistribution, GiveDirectly, is actually doing some followup research on the effectiveness of their redistribution, and it looks pretty good so far: (pdf warning) http://web.mit.edu/joha/www/publications/haushofer_shapiro_u...
That's an extreme example - a relatively-small scale transfer from wealthy donors to a much poorer country - but it speaks well to the principle.
A - Most of those attempts have had mixed results to put it mildly.
B - Scale in economics is a big deal. Obviously you can transfer wealth from one person to another pretty effectively but what happens to an economy, government, society, etc when it's the main income source is a different kettle of fish.
I didn't say impossible. But it's a technology we need to make a big leap on. Money itself is a technology. Maybe we need money itself to be disrupted to overcome some apparent limitations.
I think there are pretty strong correlations between smaller social/political units and more effective, efficient, and non-corrupt welfare/redistribution systems.
There isn't a Nordic country with a population greater than just the population of the New York metro area (let alone New York state...let alone the United States as a whole)
Even in Nordic countries "normal" is working for a living. They have big social and governmental institutions that have a lot of money passing through them and they manage to do that relatively efficiently. But, they don't have a complete disconnect between wealth creation by normal means (owning productive property and/or working) and consuming that wealth. The government is just more involved in the process.
If most people work, pay taxes and use the "free" public transport you still have a situation where most people are both funding the transportation and using it. Consumers & producers of stuff.
These futuristic ideas about AI doing all the work while most people are unnecessary creates a completely different jar of pickled fish.
If it gets to the point where there isn't any unskilled labor left to do, we can always choose as a society to vastly expand the welfare state and divvy up at least part of the accumulated surplus to everyone. We have already moved in this direction a bit, and I expect to see more things along the lines of guaranteed minimum income in the future.