This is precisely the problem with /classic -- even old (classic?) users use the main home page, so they're going to vote up stuff they see there. The front page is so path-dependent that taking out certain votes after-the-fact is never going to make big changes.

Maybe 'classic' users should be shown the classic leaderboard by default?

Alternatively, maybe classic users have the same preferences, but check the site less frequently... so they follow the newer population but at a lag.