Jumping the shark is a good way to put it for another reason: the fact that a shark's dorsal fin has this looong leading edge, and then...drops off. It's like when you've got this TV series, or group, or author, or actor... (my favorite is Sting), that has a good career, maybe a great one. They can do no wrong! You're confident, even complacent about how they'll always be there, doing that thing they do, better and better... and then... nothing. They screw up, not just a little, but bigtime. You feel embarrassed they're still around. Your old favorites are still there, but there's something... missing somehow when you know that instead of there being "more where that came from," or even "what if there were more", but that there is more. And more, and more...
Or they leave, and come back...as happened to me with Patti Smith. PS 70's? Golden. Pure unalloyed ecstasy. She leaves showbiz for anonymity in the Detroit area. Years pass, with everyone speculating on the might have been. Comeback time! Smith fans rejoice! Only now, Robert Mapplethorpe is dead, Fred "Sonic" Smith is dead, the crew is older, she's off drugs... and the wacko politics (a paen to Ho Chih Minh?) no longer seem deep or amusing.
Actual "jump over the shark tank" gestures can sometimes restart an artist by repurposing them into a different direction, but for some, it's too late.