Still doesn't mean that there is an unlimited amount of beachfront property in southern California.

Any resources not on earths surface are really expensive to get, and better AI isn't going to change the laws of physics.

I do agree with your earlier idea that mining landfills will become a very good source of resources.

When talking post-scarcity it helps to define scarcity of what. We can eventually hit a point where all basic needs, and many luxury wants are inexpensive enough to be essentially free, but that doesn't mean unlimited everything.