Can an entire TV series collapse in one episode?
How is True Detective, season 1, episode 8 not discussed in this post????
While they aren't as obvious to viewers as actor departures, I'd put forward writer departures as an equally significant factor. Many of the jump-the-shark points you found closely line up with them.
The Boondocks: Creator Aaron McGruder wrote or co-wrote every episode of the first three seasons. The fourth was produced entirely without his involvement.
Doctor Who: Having watched every modern episode, I place the blame for seasons 11 to 13 not on the cast turnover (which is continual on this series) but on the showrunner. Chris Chibnall was a far weaker writer than either of his predecessors, and has writing credits on three-quarters of the episodes he oversaw. All of the other writers in his seasons were new to the series.
Falcon Crest: Season 9 has no writers in common with seasons 1 to 8.
Master of None: Co-creator Alan Yang co-wrote three-quarters of the first two seasons' teleplays, and none of the third season's.
The Promised Neverland: Season 2 episode 4 is the last one by Toshiya Ono, who had written most of the series to that point. Most subsequent episodes are credited to a new writer, the mononymous Nanao. (Although the final two episodes are apparently so shameful that they carry no writing credits whatsoever!)
The Rising of the Shield Hero: Season 1 writing credits are divided between five people. One of those five wrote all 13 episodes of season 2 by himself.
RWBY: Series creator Monty Oum co-wrote almost every episode of the first three seasons, but died before work began on season 4.
Scream: Season 3 has no returning writers from seasons 1 or 2.
Scrubs: Wikipedia says of season 9, "every writer from previous seasons departed from the show with the exception of [Bill] Lawrence and Andy Schwartz. Sean Russell returned to write a freelance episode, just as he had done previously in season 6."
The Terror: Season 2 was created by a completely different set of people than season 1.
Great stuff. I'm still furious at How I Met Your Mother and Game of Thrones. I will randomly think, "It had to get Victoria!" and "Bran? Really? Bran?" I'm also noticing in the list that there are fewer procedurals. Is it easier to maintain quality when an expected formula is completely followed, ala the Dick Wolf universe?