I'm not sure exactly when the shark was jumped. There are so many other leading candidates for that moment.
One might have been merely the SELECTION of buffoonish James Brolin to play the GOP candidate. Another might have been the point where the Prez started receiving terse, cogent summaries of various policy points and he firmly rejected all of them in favor of knee-jerk hard-left liberal dogma. Or the shark could have been jumped when the show simply became preachy. The first two seasons had very little of that.
The remaining audience for TWW is primarily DU members and hard-left yellow-dawg Dems - those of us who ignored the preachiness and enjoyed the excellent production values have long ago left in droves.
Also, it must be said that TWW inherited a cush slot on Wednesday nights - not much in the way of competition, and the other nets haven't taken TWW on head-to-head as they might have. That will change big time next fall. Blood is in the water.
Another factor is the 9/11 pretty much made the show irrelevant. There's no way the show could match reality.
Michael
You left out another possibility: When all the GOP consultants that made sure Sorkin's wet dreams had reality checks - Peggy Noonan, Marlin Fitzwater, at least one other person - were kicked off the show because Sorkin decided they were "no longer needed."
However, you're spot on about the actual jump-the-shark moment being 9/11. In a single instant, the show no longer served its main purpose - giving Americans "a president they could trust" - because Americans finally realized they'd already had one for nine months. The real world became more interesting, and our White House turned out to have a better cast.
Other than Ari Fleischer - who I wish would be a lot more sarcastic and scathing in his press briefings a la CJ Cregg - a West Wing featuing people playing the real Bush White House staff would be far more interesting characters to watch. Especially if they made the SecDef and Secretary of State main characters.