Because "All in the Family" was never officially cancelled -- instead it was folded into "Archie Bunker's Place" -- everybody can sort of make up their own minds as to where the show ends. For some people, it ends in "Archie Bunker's Place" when it's announced that Edith has died. (The producers of "Archie Bunker's Place" were about as crass as anybody can be when dealing with beloved characters. Not only did they kill off Edith so Archie could star in his own spinoff; they had Michael abandon Gloria and his child -- utterly out of character for Michael, and an insult to viewers who had spent years watching these characters -- so that they could set up a solo spinoff for Gloria.) For others, it ends with the last official season of "All in the Family." For still others, probably most people, the show is over once Mike and Gloria leave.
For me, "All in the Family" jumps the shark at the end of the fifth season, when Mike and Gloria move out of the house. True, they only move next door, and true, it was inevitable that Meathead would eventually get a job and be able to pay for his own house. But the premise of the show (and of the show it was based on, "Till Death Do Us Part") sort of ended when Archie and Mike no longer had to live together.
More importantly, after the fifth season, there was a big staff shake-up on "All in the Family." For the first five seasons, most of the scripts were written by Don Nicholl (a transplanted British writer, best known now as the founder of a big screenplay contest that neither you nor I will ever win), Michael Ross and Bernie West (the dentist from Bells are Ringing in his previous life as an actor). Nearly all the best episodes were either written or re-written by one of these three.
After the fifth season, Nicholl, Ross and West left "All in the Family" to run the spinoff, "The Jeffersons," and a year later they would leave Norman Lear's company to run "Three's Company." To replace them on AITF, Lear turned to a bunch of veteran writers he'd known since his days in radio and early TV; they included Jack Benny's former head writer Milt Josefberg and "Your Show of Shows" head writer Mel Tolkin (one of the models for Rob Petrie, incidentally).
The new writers were all talented, experienced comedy writers, and you sort of have to admire Lear for hiring veteran comedy writers instead of considering them over-the-hill. But the new scripts were awash in comedy formulas and the kind of setup/punchline routines these guys had been doing for years. At the same time, they tried to punch up the show by making every episode a Very Special or Relevant Issue episode: Edith almost gets raped, Archie sits down to dinner with a Vietnam draft dodger, Archie gets hooked on pills. The Ku Klux Klan episode was probably the worst of the bunch. Most of Archie's lines now consisted of elaborate malapropisms and Gracie Allen-style misunderstandings. They essentially wrote for "All in the Family" as if it were a cross between "I Love Lucy" and a soap opera.
That's the way most of Lear's shows were written -- and that's why they don't hold up very well any more. But the Nicholl/Ross/West years of "All in the Family" were different, because their scripts (and the episodes Lear wrote himself) were mostly based around character comedy, around jokes that don't, in fact, read like jokes. I've harped on this before, but the best comedy writing is often not writing that looks funny on paper, but rather lines that are funny because of who's saying them and the context in which they are said. Most of the best lines from the first five years of AITF are like that:
GLORIA: Do you know that sixty percent of all deaths in America are caused by guns?
ARCHIE: Would it make you feel any better if they was pushed out of windows?
(From the menopause episode)
ARCHIE: I know all about your woman's troubles there, Edith, but when I had the hernia that time, I didn't make you wear the truss! If you're gonna have the change of life, you gotta do it right now! I'm gonna give you just 30 seconds, come on, change!
EDITH: Can I finish my soup first?
(The closing line of the episode "The Bunkers and the Swingers")
ARCHIE (to Edith): Don't you read no more magazines!
EDITH: Oh, look, Archie, Chanel Number 5! That's their highest number!
The other thing about the Nicholl/Ross/West years of "All in the Family" is that the show did very few Very Special or Serious episodes in those five years. There were a few "issue" episodes -- like the episode where Gloria almost gets raped and ends the episode by deciding not to report it, a much more effective episode than the Edith episode of a few years later. But most of the Nicholl/Ross/West episodes deal more with everyday problems, get-rich-quick schemes that don't work, arguments between the characters, and so on. They wouldn't avoid dealing with controversial issues if that was what the characters needed to talk about, but they weren't about to build the stories around iseues instead of people. Even the "dramatic" moments were relatively low-key and more about character relationships than issues. For example, the episode (written by Ross and West) where Archie disapproves of his niece dating Lionel Jefferson; the climax of the episode is partly about a social issue, but mostly about Lionel explaining the nature of his relationship with Archie, and making it clear that there's a point at which he doesn't find Archie's behavior funny:
ARCHIE: I'm saying youse guys oughtta stick with youselves.
LIONEL: You mean guys oughtta stay with guys?
ARCHIE: You know what I'm talking about, Lionel. I'm saying that whites oughtta stay with whites and coloreds oughtta stay with coloreds.
LIONEL: Look, Mr. Bunker, it's been a year and a half now since we moved into this neighborhood. I was just nineteen, and I got a big kick out of you and me for a long time. But I'm pushing twenty-one now, and I'm not getting that big a kick out of it anymore.
ARCHIE: Put a lid on it, Lionel --
LIONEL: I'm not finished. Now, we've been friends and we can go on being friends. But when it comes to black and white and all the other wonderful thoughts you have in between, put a lid on that, Archie!
That exchange, which builds on a character relationship that was established in the first episode (Lionel as the guy who subtly makes fun of Archie's ignorance and prejudice, but sort of likes him), takes that relationship to a new place, and uses a very simple device to do so (it's the first time Lionel ever calls Archie by his first name), is also a good example of the way the best sitcoms have all the character development and growth that we expect in the best dramas. Actually, in the '70s, hourlong dramas were more like anthologies, and didn't have anywhere near the character depth or development of good sitcoms.
Anyway, with the departure of Nicholl/Ross/West, "All in the Family" lost the character comedy and the organic character development. In a weird way, Nicholl/Ross/West brought some of that to "Three's Company"; it was in no wise a great show, but it did try to mine laughs from character and situation instead of setup/punchline routines, and for that reason it holds up a lot better than most Norman Lear shows, including AITF after the fifth season.