They summarize Baumol's theory for entrepreneurship as assuming that the total amount of entrepreneurial spirit remains fixed, but, for structural reasons, some of it can get channeled into unproductive rent-seeking behavior (like getting favorable regulation).
They then say that their empirical work (three decades of data so more than one recession in there) shows that there is a decline in new firm formation, "in each state and nearly all metropolitan areas, and in each broad industrial sector, including high tech."
According to Baumol, this should be offset by an increase in "unproductive" behavior. This appears to be harder to measure, but they point to [2], which argues that both labor and capital (i.e., returns to shareholders) have been in decline.
This is all very complicated from the perspective of innovation and entrepreneurial endeavors in tech, where money is often lost despite the fact that the product is useful.
[1] https://hbr.org/2017/06/is-america-encouraging-the-wrong-kin... [2]https://home.uchicago.edu/~barkai/doc/BarkaiDecliningLaborCa...