Another flaw is that humans won’t find other things to do. I don’t see the argument for that idea. If I had to bet, I’d say that if AI continues getting more powerful, then humans will transition to working on more ambitious things.
This is very similar to the 'machines will do all the work, we'll just get to be artists and philosophers' idea.
It sounds nice. But to have that, you need resources. Whoever controls the resources will get to decide whether you get them. If AI/machines are our entire economy, the people that control the machines control the resources. I have little faith in their benevolence. If they also control the political system?
You'll win your bet. A few humans will work on more ambitious things. It might not go so well for the rest of us.
There are more mouths to feed and less territory per capita for each person (thus real estate inflation in desired locations). Like lanes on a highway, the population just fills the capacity with growth without the selective pressure of even selecting for skill or ability. The ways we've come see mostly front loaded as initially population takes time to grow while the immediate low hanging fruit of domestic drudgery being eliminated was quite a while ago. Meanwhile now "work" that has filled much of that obligation in the home has expanded to necessitating two full-time income households.