As the past seasons of The Simpsons came out in sets on DVD, one had the chance to gain perspective on the show. You can see the first signs of decline already in the fifth season: that is when the characters begin to remark on the implausibility of the plots. That is not to say that the episodes are not still, on the whole, very good; just that the writing is showing strain. The writers find less and less that they can do to explore the interesting characters, so episodes start turning on the arrival of new characters voiced by celebrities and unlikely events. I think it is in the eighth season that the rot becomes really pronounced. That's when you have crap like Principal Skinner being exposed as an impostor by the "real" Seymour Skinner (voiced by Martin Sheen, I think it was), who at the end of the episode is banished from Springfield under the ruling that no one may mention what has happened ever again. By that point, you stil have some good episodes, but standards are a lot lower. I gave up watching new episodes a few years ago, but I recently saw some episodes from season 18, and I was surprised to find that they were not much weaker than the episodes of a few years earlier. But I do think that it's a pity that the series did not end when it was stil strong.