The reason SpongeBob and Loud House last so long is mainly because of corporate greed and milking by cheating through abusing their schedule and overmarketing it. (hence why all the movies and spinoffs exist)
Shows like Korra and Harvey Beaks have been sabotaged and screwed. What mind-boggles me is that MANY of them don't even last a season or two.
CN is less strict in that regard but still mistreats shows like OK KO and Infinity Train. On average, many of them last around 3-4 seasons, a stark contrast from multiple shows lasting around 5-6 seasons in the past. Currently, Teen Titans GO! is the only ORIGINAL CN show that still runs beyond 4 seasons. (most likely because of their schedule and marketing)
I know Disney has the capability to have a show that lasts long, but they simply aren't taking the risk. It's not like people hate it (Simpsons) or if it's been neglected and mistreated. (Steven Universe) Come on, Disney, even the shows that have DECLINED in quality have some pretty talented people working to make sure they can be expressive, original, and fresh while operating under corporate weight.
TLDR: Competent people are the only way for long-running shows to remain good, not milking or priority-driven directions.