[turdnagel]

Great analysis - but I also wonder if IMDb ratings have a bit (or more than a bit) of selection bias, where people rating the episodes are the "hardcore fans" who've already established the narrative the idea that the show was in decline past season 10, 11, 13 - and so since the data has potentially already been shaped by bias around a narrative, it can't reveal anything more than hardcore Simpsons fans prevailing attitudes.

In other words, you're asking a bunch of people who are obsessed with The Simpsons for their episode ratings; of course you're going to see a decline.