It went down hill after Who Shot Mr Burns.
It died when it turned out Principal Skinner was an imposter this.
The Movie was the nail in the coffin.
The jokes now are the lowest hanging fruits... so many pop culture and current affair jokes, too many easy targets and celebrity cameos. It used to be funny by itself, now it relies too much on making fun of the real world. Compare the Kamp Krusty episode to any of the modern ones for a good example.