"I think the writers lost interest in original storylines and Elizabeth Montgomery was visibly bored during filming. Poor Dick Sargent, bless his heart, was trying to fill Dick York's big shoes and many didn't accept him as Darrin."
Ms. Elizabeth Montgomery wanted out of Bewitched by last several seasons. Network kept upping the ante with money and other incentives to keep Elizabeth Montgomery and her husband (who was director or producer, forget which), to keep them coming back.
You can tell in the last year or so of Bewitched Elizabeth Montgomery was "phoning it in", and or engaging in some passive aggressive behavior. For the latter look closely as Samantha updated her Connecticut matron look from that teased bouffant to long "hippie" hair and you'll notice in some episodes under those Pucci dresses (or other outfits) she wasn't wearing a brassiere.
In last several seasons scripts from previous years were reused as well.
Audiences never really warmed to Dick Sargent as "Darrin" and ratings began to decline after Dick York left. Truth to tell even though Mr. Sargent and Ms. Montgomery were good friends in real life, their "Sam and Darrin" paring just didn't have the same magic that Dick York had. Ironically Mr. Sargent was the first choice to play Darrin, but was prevented due to other obligations.
Also you'll notice by last several seasons Endora was seen less and less, and to compensate other members of family made more appearances (Serena, Uncle Arthur), along with the Tooth Fairy, elfs, ghosts, etc.. and of course Maurice.
Agnes Moorehead was a religious woman who wasn't very keen on playing a witch in first place. As Bewitched became a hit and dragged on season after season Ms. Moorehead (a veteran actress of radio, stage and large screen) began getting restless. Rather than leave series, Ms. Moorehead requested (and received) more free time from episodes.
By end of 8th season Elizabeth Montgomery said she had enough and though plans were made for a 9th; it never happened.
https://tvseriesfinale.com/tv-show/bewitched-last-episode/
Ms. Montgomery was tired of "Samantha" and wanted to do other acting roles.
Now the dirt:
Elizabeth Montgomery never liked Dick York, and tried many times to get him "fired" from Bewitched. Depending upon who you believe Mr. York had a "thing" for his beautiful co-star but she was married to the producer, and he had a wife as well. They do say that Ms. Montgomery accused Mr. York of being to free with his hands...
In any even when Dick York kept missing work and or causing problems because of his dependence on pain killers and back injury (latter caused former), Elizabeth Montgomery finally got her wish, Dick York got the push.
Agnes Moorhead was great friends (and very fond of) Dick Sargent, so she wasn't best pleased. In turn she often made Dick Sargent's misery her life's work; on occasion reducing that grown man to tears.
When Dick Sargent was very ill with emphysema and so broke he and his wife were cleaning homes for money, the cast of Bewitched stumped up to raise funds. All except Ms. Montgomery who wouldn't give a thin dime. She never spoke to or about Dick York after he left show. Indeed a day or so before dying Mr. York telephoned Elizabeth Montgomery, but she wouldn't take the call.
Morbid trivia; Elizabeth Montgomery, her father and brother all died from cancer.
Fun Trivia: Bewitched is sometimes called the "gayest" sitcom of 1960's/1970's. Three of male cast members (Dick Sargent, Maurice Evans, Paul Lynde) were gay. Jury is still out on George Tobias (Mr. Kravits). Elizabeth Montgomery even rode in
Of course Ms. Montgomery knew about Dick Sargent and Paul Lynde. Again she had been long time friends/very close to the former, and became so with the latter. How much she knew about Maurice Evans. Ms. Montgomery was very supportive of Dick Sargent, so much so that she rode by his side (as co-marshal) at the 1992 Los Angeles gay pride march.
https://www.huffpost.com/entry/remembering-elizabeth-mon_b_7289652
While Ms. Montgomery wanting out of Bewitched is the official line most heard; it is highly likely ABC canceled or would have the show anyway.
In early 1970's (1971-1972 seasons) CBS launched their famous "rural purge" cancelling tons of show, some still popular. Other networks followed suit and so went all the kooky, supernatural, country, hillbilly, rural/country, etc....
Networks claimed their research showed viewers wanted things like gritty urban dramas.
https://en.wikipedia.org/wiki/Rural_purge
Wriggling her witch's nose, Elizabeth Montgomery was TV's sweetheart. But a new book reveals her far more salacious 'other' life.
www.dailymail.co.uk