Spongebob Squarepants, Nickelodeon's biggest success, is also their biggest downfall. Thanks to the foolish decisions of the greedy network heads, Nickelodeon's overreliance on Spongebob Squarepants has caused their animation lineup to decline drastically. Whether it be rejecting Adventure Time only to be picked up by Cartoon Network and become one of, if not, the most popular and beloved animated shows of the 2010s or causing relationships with talented creators and other crew like CH Greenblatt to sour by axing their work off the air simply because it wasn't as financially profitable as Spongebob Squarepants, Nickelodeon has shot itself in the foot numerous times and has made Spongebob their cashcow and sole life support at this point. As a result, nobody else in the animation industry wants to work for them.
What not enough people talk about is that Nick falling back on milking Spongebob may have been partially responsible for western TV animation going down the route it did in the 2010s. Because Nick relied so heavily on Spongebob from the late 2000s onwards, the battlefield of TV animation boiled down to being between CN and Disney.
CN came out guns blazing in the early 2010s with shows like Adventure Time, Regular Show, Sym-Bionic Titan, Mystery Incorporated, MAD, etc. Disney originally had Phineas and Ferb as their go-to TV cartoon, but then Gravity Falls usurped the throne in 2013/2014, becoming a phenomenon in of itself.
Then, as the decade went on, the industry became more nepotism-centered. A lot of the people working on CN on shows like AT and Steven Universe also worked on shows at Disney TVA, like GF and Star Vs. All the while, Nick was continuing to shoot themselves in the foot with how they handled The Legend of Korra, not realizing what they had till Avatar broke records on Netflix
Nick used to set trends, but their downfall really came from being set in their own ways while the world outside their bubble began to change around them. To matters even more complicated, said world ran into its own issues once Jailbreak changed everything and all of a sudden every cartoon had to be gay. Nick
could've pushed against this zeitgeist by offering an alternative like how the original early 90s Nicktoons were a raunchy alternative to the safer and softer cartoons of the 80s, but they didn't.