The only case I can present is anecdotal, which I know you'll dismiss out of hand.
Of such few people who were still playing D&D in the late 1990s, I have to assume our crew was at least somewhat representative or at least somewhat common: that being, people who started in college/university the late 1970s or early 1980s and just kept at it.
And it's the "just kept at it" piece that matters here: the odds are high those tables would skew hard towards longer campaigns and skew somewhat toward other things WotC didn't want to design for e.g. larger parties a la 1e-2e, perhaps a more West-Marches style of play, and so on.
People of high school age in 1979 or college age in 1982 would be over 35 (not by much, but by enough) in 1999. The early-80s boom time was largely driven by high-school and college/university types but for some reason WotC didn't want to hear from those people, even though that's the group to whom they seemed to want to market 3e with all the "Back to the Dungeon" advertising.