what is the evidence for the view that the U.S. business cycle has become less severe over time?

Answer 1

The major thing is the fact that we haven't had anything even approximating the Great Depression since the 1930's. If you look at the major economic indicators, just as we think a recession is looking dangerous, we go back into recovery again.

Answer 2

The main evidence is the decreasing frequency and severity of economic recessions. See the listed Wikipedia article for a review of significant recessions and panics over the years.