I use to be a big Spongebob fan when it first came on. I remember when it first debuted on Nickelodeon in May 1999. It was probably one of my favorite Nicktoons along with Rugrats, Doug, Rocko's Modern Life, Angry Beavers, etc. But since the show came back in 2005, the quality has massively dropped, and I have since lost a lot of interest in the yellow sponge.
One of the main reasons why Spongebob is in a drastic decline, is because of the fact that Stephen Hillenburg (The creator) left the show after the 2004 movie. After Hillenburg and the original writing team left, the writing for the post-movie episodes got terrible. Paul Tibbit took over the show and ran it into the ground. Both SpongeBob and Patrick got more and more stupid, Mr. Krabs got more and more greedy, and every episode started to revolve around Plankton trying to steal the Krusy Krub's secret formula.
Tom Kenny's voice as Spongebob:
Tom Kenny's Spongebob voice went from being very low key to so high-pitched and annoying to point where you want to punch a hole in the wall!
Patrick's increased stupidity:
Patrick's stupidity used to be funny in the first three seasons. Now he is so idiotic, its not even funny anymore and just plain annoying.
Nick refuses to cancel a cash cow:
Nick just won't let Spongebob go. Spongebob gets great ratings even 14 years after it has debuted, that canceling the show would be suicide. Without the yellow sponge, Nick's Ratings would plument. Lets face it, no one wants to watch all the other crap that Nick airs now.
It also doesn't help that since the mid-2000's, Nickelodeon has killed it with repeats of the same freaking episodes 8 hours a day. The yellow sponge has oversaturated the public's minds for far too long. People want variety. No one wants to see the same old thing on TV all the time.
It is funny how people criticize Scooby-Doo for jumping the shark when Scrappy Doo came on board, or how people criticize Rugrats for adding baby Dil. Yet, Spongebob has gotten a million times worse than those classic cartoons ever got, and are far better than Spongebob ever was.
And quite honestly, I would much rather Rugrats comeback and have Nick go back to the days when it was the Klasky Csupo channel, then have Spongebob shoved down my throat 24/7. At least Nick still had variety in the early 2000's.
So that is it people. If Spongebob ended after the movie in 2004, the show could be considered a classic Nicktoon. But since it is still on the air and has gotten worse and worse since 2005, it will be years before people can even consider Spongebob retro.