Decline of the generations

Twice in the past few weeks, I've heard people mention "the decline of the generations" - the idea that each generation after Sinai is a little worse than the last.

But this idea doesn't seem to me to square with common sense or with the Tanach.

If every single member of a later generation is worse, then Rashi and Rambam are worse than Ahab or Manasseh- obviously a bizarre result.

If you just assume that the generational average declines a little every few decades, we still have a problem. During the time of the Judges, Jews were massacring each other in a civil war (Judges 19-21). The Tanach leaves the impression that idol worship was quite common during the First Temple period (or at least near its end). Are either of these generations really better in any way then the generations of pious Jews following them?

More broadly: is there any way for this concept to make sense?