I'm not sure if this measure means much... with no downvotes, if quality goes down, "classic users" will simply vote less often. That would be a decently interesting statistic: (page views)/(story votes) over the lifetime of older accounts. Not sure if that data is still available, though.
Does this mean the problem at core could be with submissions and the moment it floats up to page 1 - which it would with just > 4 points in 3-4 hrs it gets reassuring votes even from old-timers.
PG, You should dig out the data on how lazy old-timers are getting in digging out material for HN front page :)