Watterson and Larson (both who retired at or near the "top" of their game) could easily have gone on for a year or two more - or three, or five, or twenty.
But they both knew that the font was running low, if not completely dry; likely triggered by starting a joke and realizing they'd done it before years ago.
Both have "come back" here and there to dabble, as appropriate for someone who actually knows how to retire.
The Simpsons used to be my favorite show, but I feel like the quality dropped dramatically after season ~13 or so. Part of that is because I got older, I'll admit, but even rewatching the older seasons, I still find them funny while season 13 and onward I simply don't.
I would have so rather they ended the show twenty years ago and use whatever budget they spent making it on new cartoons.
Schultz, who is still fairly universally beloved (including by Watterson I think?) went on forever. So did Johnny Hart (BC). The trick is that they're not really trying to get a laugh out of you every day. They're a slightly surreal setting with warmth, and a few recurring gags.
Larson and Watterson were high intensity in a way classic cartoonists weren't. That's not bad, but most people are probably going to burn out or worse (e.g. ending up like Scott Adams).
Another example of retiring at the top was Howard Tayler's Schlock Mercenary. A 20 year run of daily comics with about 16-17 years of that being a reasonably consistent storyline.
But they both knew that the font was running low, if not completely dry; likely triggered by starting a joke and realizing they'd done it before years ago.
Both have "come back" here and there to dabble, as appropriate for someone who actually knows how to retire.