Not disagreeing with you but things have radically shifted in both education and the world since 1998 and 2008. In 1998 if you had a degree you were pretty much set. It’s a much different landscape now.
The orange bars (for 2008 graduates) don't show a different result than the blue ones (1993 graduates). My undergraduate year was 2009 and, boy howdy, don't I know that having a degree isn't a guarantee of much of anything. Finding an entry-level job in that environment was pure hell, but I made it.
The typical 2008 graduate is in their late 30s/pushing 40 now, so I think you have enough data to have a good sense of how "successful" their career has been in terms of renumeration. Anything earlier than that and you might be encountering "weirdness" from an unsettled 20s or still having a good number of people in graduate school or professional school up to and even beyond 30. I'm sure you as a physician are familiar with this, but my wife didn't become an attending physician leading an ER (and have the commensurate salary that comes with it) until she was 32 (and that was after going straight to medical school after undergraduate).
Funny thing is I think the "a degree and you're set" thing was always something of a myth. I discovered a whole literature of such comments from the late 1920s and 1930s while researching for my thesis in graduate school. Yes, those were very lean times, but lean times come and go and will come to us again.