Another idiom for 'jumping the shark'

(Go) off the rails

Off the rails

Out of the proper or normal condition, off the usual or expected course.

1848 G. E. Jewsbury Let. Mar. (1892) 242 I was very worried, and I felt as if the least thing would throw me off the rails.

2007 Guardian (Nexis) 7 May (Sport section) 12 There we were bound for glory and suddenly it all went off the rails. [OED]

Go off the rails

To lose control and start to behave in a way that is not normal or acceptable M-W

As you point out, jump the shark (much less common and probably not widely known) usually refers specifically to a loss of credulity/quality in a TV series, film, book, movement, etc. on the part of viewers/readers/observers/followers often triggered by a specific outlandish event/episode. To go/run off the rails is much broader in applicability. It can conjure up the often disastrous consequences of an event as serious as a train derailment, although, as some of the Google Book examples illustrate, it can overlap with jump the shark and refer to judging a loss of credibility, quality, etc.

But the movie truly jumps the shark with its tacky, supposedly comical, finale that sees two werewolves in silhouette having sex (doggy style, naturally), and a jokey final coda that negates the very nature of the only two characters we care anything about.
...
But indeed it does offer some heart in the seemingly doomed relationship between the two likable leads—at least until it goes off the rails at film's end. Bryan Senn; The Werewolf Filmography (2017)

Then his story started to go off the rails. They had a listening device in his ear & sometimes used it to tell him things. Jeff Wade; Tales From the ER and other Places (2019)

As you must see, I disapproved. I regretted what happened. It was just one of those cases where a project by intense people who are under pressure runs off the rails. But there was no question that we had run off the rails. Andrew. St. George; Hearings, US House (1970)

No wonder the show went off the rails as soon as E. Wade's existing books had all been adapted. Olivia Dade; All the Feels (2021)

And yet I digress, since what matters to me is not really where Maya goes wrong but where filmmaking does, and I think this film goes off the rails when Bigelow and Boal carry out their belief that putting audiences in a scene helps them to understand something about it. David LaRocca; The Philosophy of War Films (2015)

Conscience is a gift. But it can become a merciless slavemaster. There is no doubt that anger at self usually runs far, far off the rails. It can become a fruitless, self-destructive routine, in the same way that anger at others can become a fruitless other-destructive obsession. David Powlison; Good and Angry (2016)

One of the girls added that even between friends, when it came to someone "going off the rails" on cocaine, for example, when someone is in obvious distress and showing signs of self-destruction: Felicia Garcia; Coping and Suicide amongst the Lads (2016)

He Crossed the Rubicon. This is from the Romans, but it is idiomatic in English.

The phrase "crossing the Rubicon" is an idiom that means "passing a point of no return". Its meaning comes from allusion to the crossing of the river Rubicon by Julius Caesar in early January 49 BC. The exact date is unknown. Scholars usually place it on the night of 10 and 11 January, based on speeds at which messengers could travel at that time.

His crossing of the river precipitated Caesar's civil war, which ultimately led to Caesar's becoming dictator for life (dictator perpetuo)... (wikipedia)

beyond the pale

To be 'beyond the pale' is to be unacceptable; outside agreed standards of decency.

The pale is an almost obsolete term for a stake or a stick with a sharp point; the site The Phrase Finder further reveals

The paling fence is significant as the term 'pale' came to mean the area enclosed by such a fence and later just figuratively 'the area that is enclosed and safe'. So to be 'beyond the pale' was to be outside the area accepted as 'home'.

Examples taken from news reports

1. BBC News: 24 May, 2022

Former justice secretary Robert Buckland told BBC Radio Wiltshire that: "If there's a deliberate lie, I can't see how anybody, including this prime minister, can continue."

"There are things we say honestly and genuinely at the time that we believe to be true... now that's one thing. Going and deliberately saying X is Y knowing that is the case is, of course, beyond the pale."

2. Sky News: June 1, 2023

Tory MP 'angry and disappointed' and describes partygate report findings as 'beyond the pale' and a 'slap in the face'

3. The New York Times: July 14, 2019

Even though Mr. Trump has repeatedly refused to back down from stoking racial divisions, his willingness to deploy a lowest-rung slur — one commonly and crudely used to single out the perceived foreignness of nonwhite, non-Christian people — was largely regarded as beyond the pale.

4. Foreign Policy.com: August 31, 2010 [all emphasis mine]

These bouts of outrage confirmed what we already knew: Blair has been exiled from polite and right-thinking discourse in Britain. He, much more than Bush, is beyond the pale; he is the man of whom we do not speak. Bush, the fashionable line insists, was an ignoramus, but Blair should have known better. […]
These days, Blair’s name is mud on the eastern side of the Atlantic. The former prime minister has been entirely disowned. He stands accused of selling his soul and, worse, his judgment to a cowboy American president and, worse still, doing it on the cheap.

The expression beyond the pale, summarises succinctly, the public's mistrust of these political leaders whose irresponsible actions eventually lead to either their resignation or failure to be re-elected.

I suggest that the then president's speech was a

pivotal moment

Collins Dictionary gives some usages:

The year 1989 was a pivotal moment in world history when one era ended and another began.

It was a pivotal moment in my life, and not just musically.

There are synonyms to be found in thesaurus.com and WordHippo such as

• tipping point
• point of no return
• turn of the tide
• turning point
• moment of truth

etc.