There are many ways of trying to answer this question. I start by running two tests, three examinations to examine whether or not the U.S. is a less competitive environment. The first is what percent of the total market cap (AMEX, NASDAQ, NYSE) is held by the largest 10 and 20 firms (click to enlarge). The second is what percent of the total market cap of the top 100 firms are held by the largest 10 and 20 firms (click to enlarge). Both show fairly clearly that inequality, in the top 100 (and across the market barring market size effects) has reduced: any "capital" advantages held by the largest firms are smaller than they were historically.
Next I look at the 10-year change in the top-10 and top-20 firms. That is, the chart below (after 1935) gives data on the number of new firms in the top 10 or top 20 (by market capitalization) that weren't in the top 10 or top 20 10 years ago. Unlike the cross-sectional data on firm size, this shows a lowering of competitiveness starting around Q2 2001 and continuing for the next 13-14 years (click to enlarge).