I thought I’d never see a definitive answer to this one, but thanks to the brand-new Google NGrams Viewer, the facts are clear:
A lot of words and phrases (though not these) show a dip starting in 2000 or so. I wonder if the nature of the corpus changes at that point to include more words? You see the same effects with name frequencies — the frequency of any given name has been decreasing over the last twenty years, just because names are getting more and more widely distributed; the most popular names today take up a smaller share of namespace than much lower-ranked names did in the 1950s. A quick and dirty thing to check would be the entropy of the word distribution; is it going up with time?
Oh, and here’s the Four Shortstops:
Ripken, appropriately, is showing great staying power.