I was around when the oldies format started in the late 60's and early 70's. The first oldies station I heard was WHYL, then it moved to larger FM stations like WCAU-FM and WCBS-FM. The format was never meant to be static and lost in a 50's time zone.
Programeers originally perceived that to retain an audience, you had to adjust the years you covered. In 1971, you basically covered 1955-1963. Each year you would progress. By 1973, you would have dropped 55 & 56 and added more British Invasion and other Top 40 hits from 64 and 65. But as disco and hard rock changed Top 40, most programmers dug in and froze the format to primarily 1959 to 1972. The newer music genres just did not mix with the early rock and roll, or so programmers thought.
By being selective, you can easily produce a playlist that runs from 1964 through the 80's, but it takes effort. And you have to do it. Today's audience considers songs from the 80's to be "oldies." Which, let's face it, is true if you were born in the early 70's. To keep your audience fresh, you have to keep the playlist "fresh".