I guess me and all of my friends were sporting hipster affectations when we were 12 years old because we all noticed the change around 1999-2000 and correspondingly stopped watching. Interesting that I also had my VHS tapes of earlier seasons on repeat from that time until I learned how to pirate copies with XDCC on IRC.

The burden is on you to prove that the show hasn't changed. If TV shows didn't change they wouldn't be cancelled. Writers change, plot lines get exhausted, the network demands for a cash cow vs a dynamic new comedy influence the style. That's not even mentioning the famous quote by Mike Scully (show runner, conincedentially taking control right at the moment of supposed quality dip):

"When asked in 2007 how the series' longevity is sustained, Scully joked, "Lower your quality standards. Once you've done that you can go on forever."

I almost regret putting so much effort responding to a comment that contains troll bait like "the simpsons sucks, because it has always sucks" but the simpsons is a truly special show and what it was deserves to be recognized.