You will find this question discussed in a symposium at Econ Journal Watch, co-sponsored by the Mercatus Center. Contributors include Richard Epstein, David R. Henderson, Richard Posner, Daniel Houser, James K. Galbraith, Sam Peltzman, and Robert Solow, among other notables. My own contribution you will find here, I start with these points:
If I approach this question from a more general angle of cultural history, I find the diminution of superstars in particular areas not very surprising. As early as the 18th century, David Hume (1742, 135-137) and other writers in the Scottish tradition suggested that, in a given field, the presence of superstars eventually would diminish (Cowen 1998, 75-76). New creators would do tweaks at the margin, but once the fundamental contributions have been made superstars decline in their relative luster.
In the world of popular music I find that no creators in the last twenty-five years have attained the iconic status of the Beatles, the Rolling Stones, Bob Dylan, or Michael Jackson. At the same time, it is quite plausible to believe there are as many or more good songs on the radio today as back then. American artists seem to have peaked in enduring iconic value with Andy Warhol and Jasper Johns and Roy Lichtenstein, mostly dating from the 1960s. In technical economics, I see a peak with Paul Samuelson and Kenneth Arrow and some of the core developments in game theory. Since then there are fewer iconic figures being generated in this area of research, even though there are plenty of accomplished papers being published.
The claim is not that progress stops, but rather its most visible and most iconic manifestations in particular individuals seem to have peak periods followed by declines in any such manifestation.