Law of averages: is 50-plus beyond most current Test batters?

Historically, a career average of 50 in the five-day game has been the gold standard that batters have aspired for. But since 2020, only three have achieved the benchmark whilst scoring a minimum of 2000 runs, in contrast to 19 from 2000 to 2009 and 11 from 2010 to 2019. Why this decline?