Probably what happened is America was in an increasing returns to scale part of its production function. Now it's in the diminishing returns to scale portion. If I had to guess why, it would be due to growth boundaries of cities and lack of new cities. There's no new Manhattans. Instead Manhattan has just gotten more and more expensive to live in.
Anyway, if F(Population) is the production function, then wages should be F'(Population) and total wages should be Population.F'(Population).
F(Population)-Population.F'(Population) is total production minus total wages and is known as economic rent.
The right thing to do would be to tax economic rent. Use a tax on land value and natural resources to fund UBI.