On the subject of All That, I'd say it jumped the shark when it became "The NEW All That", when pretty much the entire cast was replaced... but then again, that's like all these old codgers and middle-agers who say, "Oh, Saturday Night Live was the never the same again after the original cast"... but I mean, without the likes of Kenan and Kel, Amanda Bynes, Lori Beth Denberg, Danny Tamberelli, Josh Server, and them, the show just wasn't worth watching.
As for Seinfeld, I feel the reverse, I feel the first three and a half seasons jumped the shark, if only because there was so much network scrutiny, too much network control, not enough creative freedom, etc. By the latter half of Season Four and onward is when the show kept getting better, and with a few exceptions (such as the episode that plays backwards), I think the last couple of seasons were the best... of course, that could be because that's when I remember first watching the show (I was only a kid when it first came on), so that could be why those episodes feel stronger to me than earlier ones. Yes, Larry David's departure was a blow, but there's a number of series that still go strong after the creator walks away: M*A*S*H went on for seven more seasons after creator Larry Gelbart left, even though the last four seasons were pretty bad.
Speaking of which, I agree with Daniel that the departure of Radar was bad for M*A*S*H, but I think with the direction Alan Alda took the writing and production staff in those last few seasons, that had Radar remained, it probably wouldn't have helped much. I do disagree a little on Frank Burns, I think Charles added new blood to the show, but he was also like Frank at times in that he was a character you could love to hate, not because he was a weasely twerp like Frank was, but because of what a heartless rich snob that he was, thinking he was better than everyone else in camp, yet every so often, Hawkeye and B.J. could pull the wool over his eyes without him realizing it (ala changing his uniforms to make him think he's losing weight, then gaining weight, etc).