Well first of all, sitcoms are largely garbage. The canned laugh track, the moral lessons, etc. is brutal, once you reach a certain age/level of maturity. They're harmless, mostly, sure, but for the talent, it's a paycheck.
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Secondly, they all have some sort of bell curve where they peaked. If you look at the first season of the Simpsons, it's worse animation and worse voices than you'd think. And it was less sharp-tongued and just crude and crass. After a few seasons, it became what we think of it as being, and that lasted for a time - maybe 6-7 years.
One rare thing the show did was cycle through writers and stayed strong for a time. It didn't fall off a cliff after the first group cycled out. I think that's when most shows fall off.
