I think Once has jumped the shark because the quality of recent seasons has been nowhere near the quality of the first two seasons.
But that’s not what a shark jump is. Any show on a given timeline will decline with the first few seasons being the best and the last few being less than (in varying degrees) those first ones. But that doesn’t mean they’ve jumped the shark. Jumping the shark is when writers use an outlandish storyline or stunt or gimmick that is meant to bring in ratings, eyeballs and money from advertisers that will save the show from declining to the point of cancellation and keep the viewers still tuning in interested. It allows the show to slog on for a few more years. That’s why it’s called jumping the shark–because that was an actual gimmick performed on an actual show with the intent of helping the show stay afloat for a few more years (Happy Days; the Fonz jumps over a shark in S5).
But writing that does not hold up to snuff as compared to writing that used to hold up to snuff does not mean there has been a shark jump. (This is to say nothing of the fact, of course, that some shows just go through a rough patch and then rebound. For example: Supernatural. Most fans/critics will agree that S6 and S7 were nowhere near the level of greatness that S2-5 were. But they also agree that the show managed to come back quite strong in S8-present day, S11).
When we talk about OUAT jumping the shark it’s not enough to say that the writing has declined. Of course it has–but so did the writing on the West Wing, Mad Men, The Soparnos, and even with a show still on the air like Game of Thrones and The Walking Dead. But that doesn’t mean that any of those shows have jumped the shark.
But to say that OUAT jumped the shark is to point to a specific storyline or event/stunt that was meant to save a floundering show to ensure that it could go on for a few more years. Follow the money, guys. When did OUAT suddenly get more $$ from ad buyers? Season 5. What came right before? Season 4. What was S4A? Frozen–the opening episode of which got the highest Live rating the show had gotten since S2 by using a media product (Disney’s Frozen) that earned more $$$$ for the company (Disney) than any other movie ever. Intent is what matters here. Intent of the writers and the network.
This isn’t to say that A and E didn’t really love the movie Frozen or that the OUAT S4A didn’t occasionally do something well (like the episode, Shattered Sight). But that’s just icing on the cake for the writers/network. The intention was to get high enough ratings/money from the ad buyers to keep the show going. And it succeeded.
t is not something that is intentionally done, unless the writers are deliberately trying to screw over their show, so it will get cancelled. Whether or not Adam and Eddy are doing that is something we could debate for ages. I, personally, think they’re just that bad of writers.
It is absolutely something that is intentionally done. The writers do not sit down and say “hey let’s have a shark jump” but they do think about some sort of gimmick or plotline, that will keep people watching and bring back those lost eyeballs. Again, think about Frozen. We know A and E had an entirely different ending planned for S3B that got canned because they got permission to do Frozen. They jumped at being able to use Frozen because they knew it would bring in those eyeballs.
The writers on Happy Days did not having the Fonz jump over a shark cause they thought it fit with the show and made sense. They did it because they knew people would watch that event and (hopefully) continue to watch afterwards. And they did. It lasted a few more years after the shark jump.