That’s because Levin is an evil feminist who uses his novels to turn women against men, portraying husbands as evil, toxic manipulators. He’s trying to dissuade women from becoming wives and mothers as part of the Communist goal of destroying the family structure.
Gotta give him credit though - he’s a master of this kind of conspiracy horror.

The element of betrayal at the end gives it an extra emotional gut punch. The whole fact that Joanna was betrayed by her own husband – a man that she basically loved and trusted or at least once did – is what makes the ending so chilling and upsetting. The same thing happens to Rosemary at the end of Rosemary's Baby. So the theme of betrayal is a constant one in Levin's work.
You've seen Guy Standeven in something because the man was in everything.

I'm also happy that it ended the way it did.
It's basically the way Ira Levin wrote it. No need for any changes.
You asked a pretty question; I've given you the ugly answer.
Fasten Your Seatbelts….
It's Going To Be A Bumpy Night!

I love that this movie didn't punk out with some cliché and contrived Hollywood happy ending. This film certainly has the strength of its own grim convictions as far as the central premise is concerned.
You've seen Guy Standeven in something because the man was in everything.

The ending is perfection.

Perhaps she was still trying to register it all.
Even after stabbing my friend and seeing what I had seen, I am still not so sure that I could comprehend the sight of this robot who looks like me even BETTER than me in a carbon copy of my bedroom. I'd be a bit stunned by it all. I'd still be reeling at the thought of such a betrayal on my husband's part.
"Fasten your seatbelts. It's going to be a bumpy night"

My problem with the ending was that Joanna just seemed to give up when she spotted the robot. Why wouldn't she have at least tried to fight it off? It didn't seem true to her character to just accept what was happening.

Perhaps the ending could be read as the failure of feminism.
Exorcist: Christ's power compels you. Cast out, unclean spirit.
Destinata:

This ^^
We urgently need a blu-ray but the rights are owned by a drug company (who also owns Sleuth) and they refuse to release it.

It's like the popular story "The Lottery." Or the movie "The Wicker Man." An insular, secretive community that's hiding a pitch dark secret. And, an innocent, or innocents, are doomed to be taken down by it.
Ira Levin devised the most diabolical horror plots ever. And, he likes to ride them to their end. This book/movie follows that trend.
[SPOILERS]
It is so completely evil! And, I so completely love it! It's one of those cinematic moments where my tears are welling up. Not because of sadness. But, of joy at how cinematically perfect the moment is. Beautifully freaky, including Owen Roizman's cinematography, and Dick Smith's makeup work on Katherine Ross. I so wish this would get a restoration and a Blu-ray.
Plus, the fact that the robot is the one who kills the original is just absolutely twisted.
For those wanting a "happy ending," you get it! It's just a purely EVIL happy ending, as shown in the still photos of the "idyllic" new Katherine Ross with her family.
I. Drink. Your. Milkshake! [slurp!] I DRINK IT UP! - Daniel Plainview - There Will Be Blood

I’m not a mother, I’m not even a woman, and it’s patently obvious to me that no parent would abandon their children to some cult of grubby men who appear to be turning their wives into sexbots.
This raises an issue with the plot - won’t the children of the sexbots be weirded out by their mothers becoming uncanny cyborgs who don’t age..?

True. After having a child, I could never leave my child in that situation.
You asked a pretty question; I've given you the ugly answer.
Fasten Your Seatbelts….
It's Going To Be A Bumpy Night!

Plus unrealistic. No mother would just ‘leave the kids’.

Personally, I thought the ending was perfect.
The book and the film had basically the same end.
I use to think Joanna was foolish to come back to town after seeing the psychiatrist. She should have just driven away instead of coming back for the children. They would have been just fine.
She could have stayed with friends in the city and eventually filed for divorce and custody of the kids
Nah! Not a good ending for a great little thriller like this.
"Fasten your seatbelts. It's going to be a bumpy night"

The film implies from the start that the wives are real women. The robots are only temporary.
Whether they could be half android as well is certainly possible.

What if Joanna damaged her android replacement in the struggle? Just enough that fixing it would be to costly and starting over take to long. Diz makes the decision to take human Joanna and turn her into a half human/half android. She is still under the control of Diz in this new form thus making it worse than if she had died. The look in her eyes at the end of the film would make sense as she is still human though under the influence of her android programming.

There always, always has to be a woman to find pretentious hidden meanings of females being wronged, feminism etc., and also cite the 70's as some target also. The year is was made is irrelevant
Ira Levin, a guy, wrote the damned thing. The year the book was written and the year the film was made is very relevant. Feminism was a large part of the story. Men, at that time were feeling a loss of control.
It isn't all that relevant today. That's why that awful thing they call a remake didn't work.
Yet, when men are defeated and doomed by women (or men) in a film, there is no reaction. Women like the OP are venting their own demons, I think. And over a harmless horror film/thriller
Well, off the top of my head, I felt the same sense of sadness at the end of The Mist.. Another "harmless horror film/thriller".
"Fasten your seatbelts. It's going to be a bumpy night"

There always, always has to be a woman to find pretentious hidden meanings of females being wronged, feminism etc., and also cite the 70's as some target also. The year is was made is irrelevant
Yet, when men are defeated and doomed by women (or men) in a film, there is no reaction. Women like the OP are venting their own demons, I think. And over a harmless horror film/thriller

This ending was terrifying. As it is supposed to be.
It worked for me.

Because when I think of cautionary tales, I think of, "Don't worry! Everything will turn out alright in the end!"

Downer endings are for adults. So many authors have satirized the cliche of the "coddled, insecure public that can't stand a harsh ending and needs a simplistic, happy Hollywood outcome," and I like to think people are tougher than that. But yikes, threads like this just show how pathetically true that particular cliche is.

Makes sense.
And like you said, it's a horror flick, and the downer ending was part of it. I now just realized that it's not saying that "Feminism will fail". It ends grimly because that's SCARY. It's just one town, anyway.
Besides, there was a sequel where the "Men's Association" get their comeuppance. The two afterwards are just milking the concept to stupid degrees. I like to pretend that only the first two movies are canon. There's the "Feminism vs. Misogyny" theme and the fact that it allows the bad guys to lose in the end. I like that.
I need to quit getting sensitive about bummer endings and not see every flick as a big social message.and just take each movie on a story-by-story basis.
For me, I'll just check out "spoiler" websites to ease the sting of a grim ending.
..and get into fanfic writing for relief.
After all
"You don't have to accept the ending they give you." - Joel (To Crow & Tom Servo on MST3K)

Well, it's a horror film, first and foremost. The subject of feminism is really used to drive the plot, as when Joanna says to Bobbi "I messed around with feminism in NYC" ("who hasn't", I believe, was the response), and "I'm not talking a Maidenform Bonfire here, but", and then, in reaction to the alarm they feel at what they see in Stepford, they go around trying to raise consciousness.
But if you really need to make the ending meaningful, in a (somewhat) positive way in regards to feminism, how about "if we women don't stick together, bad things can happen". The men used a "divide and conquer" scheme to change the wives, as when Bobbie went away and never came back. Just spitballin'
Takes two to tumble it takes two to tango
Speak up don't mumble when you're in the combo

The ending has nothing to do with all that, the 70's and so forth ; many thrillers films end in doom and gloom. Any other films where you wish to cite false misogyny and feminism?