We invent new things for people to do. As we have time immemorial. What happens to clergymen when nobody has faith in a religion? They find other things to do than preach.
> What happens to power structures when people become a cost rather than a benefit?
If you want to look at this pessimistically - a good portion of the world's population can already be considered a cost and are a net economic negative on the privileged people's ability to hoard even more insane amounts of wealth. We've, thankfully, decided that mass genocide of impoverished people is inhumane and humanitarian efforts are instead created to try and improve their lives. Even a pessimistic take that the humanitarian efforts only being done to create more workers doesn't hold up given a cost/benefit analysis. More money is invested than could ever be extracted again - and yet we continue investing.
Maybe instead of filling our lives being paid to dig holes so that others can be paid to fill them we could turn more attention and efforts to improving people's lives without having to worry about sustaining society through work?