It's also possible that the average quality is decreasing but the quality of those that reach front page is increasing

Possible, sure. But likely? Increased submissions with no change in average quality can fully explain an increase in front page quality, as can increasing average quality, of course. And scores aren’t a quality metric, they’re are a popularity metric. The decrease in scores can also be fully explained by increased submissions. So there doesn’t seem to be any reason to suggest quality is decreasing…