Genuine question - what does that phrase mean to you?
I've only ever heard it in the context of television shows (meaning "has been renewed for so many seasons that the writers have run out of ideas and have started forcing the characters into ever wackier situations to generate novelty") - and while it's intuitivve to map from that meaning to a more generic "has gone on for too long and thus become bad", the translation is fascinating. When you said this, were you intentionally using a term from another domain with a different meaning in the confidence that your audience could translate it appropriately, or does the phrase "jump the shark" really just mean "be old, and thus bad" to you?
(Asking from a purely non-prescriptivist non-judgemental perspective! I'm interested in understanding your thought process, not in judging or correcting you)