Every sitcom that devolves into something awful over the course of its lifespan usually comes to rest on one of several predictable structures:
- Some idiot always tells lies
- Some crazy person always acts wacky
- Some unfulfilled sexual tension persists indefinitely
It's remarkable, in fact, because it happens so consistently, usually after several seasons, when the series no longer has any momentum at all, but its previous seasons have given it such a large audience that the network is desperate to keep it alive. This is usually the shark-jumping season, where every episode features somebody getting married, having a baby, or hugging for some reason. Of course one of the rules Seinfeld laid down for his show was "no hugging." Good rule.
What happens in Seinfeld is, rather than one of these three possibilities being the inevitable devolution, all three constitute the cast. George is the liar, Kramer's the crazy person, and the persistent unfulfilled sexual tension comes from Jerry and Elaine. The last one is least obvious, but I think it's accurate, because imagine if you were dating either one of them and you met the other. It's very possible you'd rationally conclude the relationship had no chance, because look how much time this person spends with their ex. Kramer even blows up at them in an episode, saying, "Can't you tell you're in love with each other?"
In medieval Italy, there was this concept of the commedia, a consistent set of characters performing a variety of dramas. The characters of the commedia were the archetypes of medieval theater. There was the fool, the lecher, the hero, etc. I think what really happened with Seinfeld is that the show established an equivalent to that system, where they had a set of archetypes of the modern sitcom, and were able to run them through any dramas they wanted, because their roles were so predictable -- and because these predictable roles are the roles that keep sitcoms alive.
It's the second point that really matters. Seinfeld's casting short-circuited the inevitable devolution any successful series experiences by making that devolution the starting point. You could say they got lucky; you could say they sensed it intuitively. You could even argue that these statements are essentially identical. It doesn't matter. What's worth keeping in mind is that this show was hugely popular when it went off the air, and it may be that it avoided jumping the shark by consolidating all the features that are characteristic of a sharkjumper in its cast.