>>pg+e
The decrease in quality could be due to less good stories on the new page and not newer people upvoting. That is, newer people may be just as good at upvoting, but the real issue is that so few people upvote from the new page. If true, this change would further the problem because even less real votes from the new page would be occurring.
To test this possibility, randomly reserve some of the places on the front page for new stories (perhaps in the bottom half). Then more people would be looking at and potentially upvoting the good stories from the new page.