York left because of chronic pain problems due to issues with his spine. He'd become somewhat dependent on pain medicines, and his health was making it difficult to continue in the roleas you note, he didn't die until a decade after the show had left the airwaves.
50 Is The New Cutoff Age.

That would have been a hard one to pull off since York had passed away at the time.
Dick York didn't pass away until 1992.

they SHOULD have had Dick York either back again
That would have been a hard one to pull off since York had passed away at the time.

I know this is through hind-sight, but lately I have felt that they blew it on
Bewitched
when they changed Darrins.
In the very beginning of Season 6, episode 1 they SHOULD have had Dick York either back again for a short scene where he gets changed into something or someone else by a goofy witch or warlock (Aunt Clara or Uncle Arthur), OR they could have used a sequence from an earlier episode. (I know they did that at least once before.)
ANYWAY, time comes to change Darrin back, and Clara/Arthur can't do it at first. When they finally DO change him back - voila, it's Dick Sargent.
Of course, Darrin is VERY upset about this but Sam assures him she still loves him and he'll get changed back soon (he isn't of course). In the meantime, Sam casts a spell on all humans who know Darrin which makes them think he hasn't changed appearances at all.
The biggest problem to all of this is that Dick Sargent would have had to of been a MUCH better actor, in order to continue to emulate the mannerisms of Dick York. I doubt he could have done it, though.

That time on The Waltons when John-Boy came back from the war a completely changed man. Literally
Most incredible case of PTSD on record.
I agree; Wightman might have been an okay actor, but he was being tapped to fill an iconic role and didn't have what it took to pull it off. I'm not sure any actor of that era could have, really.
50 Is The New Cutoff Age.

That time on
The Waltons
when John-Boy came back from the war a completely changed man. Literallyhe was played by Robert Wightman, not Richard Thomas. The show was losing steam by that point anyway, but that was the end for me. The new actor didn't even have the mole!
Years later, he recovered enough from the war to revert back to Richard Thomas again.
I'm not crying, you fool, I'm laughing!
Hewwo.

It's weird to watch the early Seinfelds with a totally different Morty (his dad). The original guy was pretty boring and not Jewish enough, but I never cared much for any of the scripts focusing on Jerry's parents.
In The Munsters, Marilyn Munster was played by two different actresses: Beverley Owen (in the first 13 episodes) and Pat Priest (in the rest of the series). Marilyn was recast yet again for the follow-up theatrical film, Munsters Go Home with Debbie Watson.

I really don't care I can't remember any show that has bother me actors changing. The Dallas show. It was kind of weird not seeing the same actress. I didn't know about the lawsuit!

Chuck Cunningham after Harvey the Rabbit replaced the original actor

Bewitched again with the Darrins

I may prefer one actor over another, but it never bothered me too much. I grew up watching daytime soaps where recasts are a way of life.
Four actors played Harry Morton on
The George Burns and Gracie Allen Show
. George would break the fourth wall and speak to the audience on that show. Before the last Harry, Larry Keating, made his appearance George explained to the audience that Fred Clark had left to do a play and introduced Keating.
"Ain't it funny how coffee never tastes as good as it smells?"

Becky in Roseanne