Replying to my own post: wait a minute, why are there so many accounts with the same password in the first place? Oh, because "dozens" of people are tired of changing their password every 90 days, and someone piped up on an email thread (with the subject line: "Changing passwords all the time is bullshit!", I'm sure) and said, "I just set it to $SEASON$YEAR'!'. Easy to remember, fits the policy."

And now you have a system that is far less secure than if you just ditched the expiration policy to begin with.