This is one of the things I dislike most about ST is that almost no one who is a main character who dies, ever stays dead.
...
It's pretty anti climatic when so many people are killed but find ways to live on. Death is part of existence. Kill a character and say "That's it folks".
Welcome to how the soaps do things. Some characters get shot, blown up, die of a disease, fall down a bottomless pit, drown, or get killed by lots of other ways... and yet they still come back. Just last Thursday, one of the longtime characters on
General Hospital was shot and died of his wound. Yet the fans are already speculating on when he'll be back, since hardly anyone on that show dies forever. It takes the actor's RL death to even begin to make it permanent, although of course TPTB can always recast. Or they can just do what
Dallas did and have the character show up in the shower a year after he was run over and died, and his wife says, "Bobby, I had the strangest dream..." (and thus Dallas canceled out one of its best seasons plus messed with the storyline on Knots Landing and so the two shows that used to be connected ended up in different continuities). Or they go with the "identical twin/cousin" thing so the actor stays on.
So Spock dying in TWOK and coming back in the next movie is really not that much of a stretch. At least it had a semi-plausible in-universe reason for it to happen, plus a way to make it happen.
Well to compare your statement that having Patrick Duffy's death ruin a really good season by having all be a "dream" by Victoria Principal. Look at Spock in TWOK.
I'm not going to say the fact they brought him back in TSFS ruined TWOK or his heroic demise.......because it's still a great film and scene.
It sure as hell made it less powerful and emotional though. I mean imagine every time you watch the scene between him and Kirk you know it would be the last time you'd ever see Nimoy as Spock on screen. I think no matter how many times I saw that scene I'd get choked up if that had happened.
As it is, yeah it's a great scene and moving and all, but I don't exactly feel sad watching it because I know that Nimoy came back alive and well as Spock in 6 films and a 2 part episode of TNG after his "death" in TWOK.
There's one crucial difference you appear to have missed in my Dallas reference (aside from it being that Bobby Ewing died, not Patrick Duffy; he's still alive). Bobby Ewing was hit by a car and died. The next season dealt with Pam and Jenna dealing with their grief, and JR throwing himself into new business ventures with some mysterious European ladies. The show introduced new characters Jack and Jamie Ewing; Jack became JR's new 'sidekick' and business partner, while Jamie had a somewhat stormy relationship with Cliff Barnes. The overall storyline literally went to places the show never did before, and for the first time in years it was a refreshing change from the same-old-same-old stuff. And over on Knots Landing, Bobby's death was written into
their storylines and had an effect on how they played out.
Fast-forward to later, after Patrick Duffy decided he wanted to come back... and Pam gets up one morning and there's Bobby in the shower. End of season. The following season we find out that the exciting, interesting season we'd just seen was nothing more than Pam's dream.
What a cheat. If Patrick Duffy wanted back, they should have made him an evil twin, or even some nonsense that Bobby had really survived but his body was stolen/he went into some sort of witness protection/whatever other soaplike excuse
not to undo all that European storyline and mess up Knots Landing's storylines.
At least in TWOK/TSFS, things were more linear and they did find a plausible reason why the Spock they ended up with at the end of the third movie was really the Spock who died in the previous one. They didn't have Kirk wake up one morning, go to the Bridge, see Spock, and announce that he'd just had the strangest dream (although I suspect there may be a fanfic somewhere that depicts this just like in Dallas, shower and all

).
Thing is, I rather doubt the writers/producers of
The Search For Spock worried much about that. They wanted to find a way to bring Spock back and they did it as the major part of the story, not some 30-second throwaway nonsense that rendered the previous movie irrelevant and "non-canon".
You're right in that Spock's death seems less impactful story-wise
now, because of course we know that he came back and was eventually just fine. But the scenes themselves are no less poignant. It's like some of my favorite novels in which beloved characters die. I've read and re-read them numerous times, and sniffle and cry every single time.
Mind you, these characters tend to stay dead unless they're ones in the fantasy series where they come back in some alternate dimension or afterlife. But that doesn't alter the fact that their original death scenes were tear-jerkers.