I believe 'SpongeBob SquarePants' is one of the best cartoons ever, especially for the first 3 or 4 seasons. I don't think I really need to explain why it got worse after a while as there are several videos about why it went downhill on YouTube. The gist of those videos is that the late Stephen Hillenburg and some from the first three seasons left the show, and then new writers came in and that's where the show lost its charm and wit. While the show didn't immediately go down in quality, as there were some pretty good episodes of Season 4, there were also some really bad ones. Some of the episodes of Season 4 feel like the Season 3 episodes, and I even thought they were a part of Season 3 when I was younger. In my opinion, good episodes from Season 4 are: ‘Fear of a Krabby Patty’, ‘Shell of a Man’, ‘The Lost Mattress’, ‘Krabs vs. Plankton’, ‘Skill Crane’, ‘Have You Seen This Snail?’, ‘Krusty Towers’, ‘New Leaf’, ‘Selling Out’, ‘Mermaid Man and ‘Barnacle Boy VI: The Motion Picture’, ‘Mrs. Puff, You're Fired’, ‘Best Frenemies’, and ‘Dunces and Dragons’. All are good episodes of 'SpongeBob SquarePants'. If the post-movie ‘SpongeBob’ was more like these episodes there would be not as many problems with post-movie ‘SpongeBob’.
Now I would have ended the show after the 1st movie because it was the perfect ending and was supposed to actually be the ending. But due to the high ratings and success of the show, it would be dumb for Nickelodeon to end a popular show. Also, the characters became trait shells of their older selves. For example, Mr. Krabs was cheap in the old episodes but in the post-movie episodes, they took in up to 11 for no reason. Also, they made SpongeBob who used to be an adult who acted childishly. Into an annoying mess. Patrick used to be dumb but a kind friend into a mean jerk. The only characters that didn't change much were Squidward and Plankton. In fact, Mr. Krabs is more of a villain than Plankton was in these awful seasons. So, it went on season after season bad episode after bad episode. The reasons why there bad are they use gross humor, made the characters mean and not likable, and lost what made the old episodes so good. Also, some of these episodes are so bad I question why this was this created for kids. Like the episode ‘The Splinter’, for example. I mean this doesn't even remotely resemble classic ‘SpongeBob’. The old episodes were creative and funny and were shown kids and adults can enjoy it. The post-movie episodes were for the most part just for little kids and not for everyone. They feel more like parody fan fiction.
Also, since ‘The SpongeBob Movie: Sponge Out of Water’ came out, Stephen came back as a writer for the show. While I'm not the biggest fan of the newer ‘SpongeBob’ and haven't seen much of it, many say it's a huge improvement over post-movie ‘SpongeBob’. Also, there is going to be a new spinoff of the show called ‘Kamp Koral’, and actually I don't know why they made this. One thing Stephen Hillenburg wanted was no spinoffs of his show. And if he was still alive, he wouldn't greenlight this show. Second, it feels late to have a spin-off of ‘SpongeBob’. If this was made in 2005 or 2006, it would have made more sense, it was when ‘SpongeBob’ was at its peak of popularity. Which, in my opinion, was from 2002-2012 AND, it destroys the continuity of the show I thought SpongeBob first met Sandy in the episode ‘Tea at the Tree Dome’ in Season 1, and the same goes for Plankton they first met in the episode ‘Plankton!’ of Season 1. It feels like a cash grab. But who knows? Maybe the show will be amazing.
In general, I'm glad this show was created as it was funny and clever at its peak. RIP Stephen Hillenburg for his amazing show and created millions of people's childhoods.