An interesting question, but I think theoretical at this point. The evidence is very strong, not only in the U.S. but in other advanced economies who all perform similarly. In addition to the evidence, expert opinion (economists) seem unanimous; that is, I've never heard one say otherwise.

Fundamentally, the size, i.e., the output of the economy is easy to compute: Inputs (resources, such as labor and capital) x Productivity (how much benefit is wrung from the same resources).

The inputs generally increase. One major input that increases is labor. As the population grows, there are more people doing productive work. Productive workers, which is almost everyone with a paying job and very many without one (homemakers, volunteers, etc.), produce more than they consume - an important point. It's not a zero-sum situation; adding people doesn't reduce everyone's slice of the pie, it makes the pie bigger.

Productivity also increases. You see it in SV every day, with better and better software, as one simple example. Every improvement in knowledge and technology, in every field, adds to productivity. Is there a limit to productivity? It's hard to imagine, but if there is then we're nowhere near it.

> Everyone is very highly incentivized

Or maybe the evidence is very strong. For example, is everyone is incentivized to believe the theory of gravity, or is the evidence very strong?