I feel like there would have to something significant in the style change of the show to be 'jumping the shark' though, not just the writing going down. s5 and s7 are my all time favorite, and my second favorite episode is in s10 (The Gang Misses the Boat), so would disagree that's when it started to go downhill, s4/s5 seems to basically be majority consensus as their prime, including them.
I would personally say they arguably jumped the shark in s13 , when Glen wasn't sure if he was coming back, and then Rob getting into his other shows/buying a football club. I think Mac Finds His Pride was super close to a jump the shark moment, it was way too pretentious to close out their arguably most mediocre season. But honestly every season has classic Sunny episodes that still meet the bar imo, s13 still has The Gang Gets New Wheels and Time's Up For the Gang; there's probably nothing from s14 or s15 I love as much as those two, but s16 was their best in years.
Big Bang Theory and Two and a Half Men*
Friends has it's place, and some truly iconic moments in pop culture. Even if the writing doesn't fully hold up, the chemistry there was legendary for a sitcom
Again, I'm using the term loosely to just mean a drop in quality into totally mediocre. If it's technically the wrong term to use, I'm happy to agree that I'm using hyperbole to express a general sentiment.
I think season 5 is still pretty enjoyable overall, but I do feel the start of a decline in what I liked about the show, personally (I think season 4 was pretty air tight and inspired). It starts to feel zany/gimmicky with less cohesion/organicness, despite still regularly being funny, in my opinion (they're all zany and gimmicky to some degree obviously, but I think it really consistently worked and always felt inspired rather than forced before that point). It's similar to how I feel that season 5 of The Simpsons is also the start of its decline, despite tons of people loving it/thinking it's the peak and a ton of episodes being considered classics. I just disagree with them.
I can agree that stuff like S13 is when it gets really unwatchable (weirdly similar to The Simpsons as well), but I was already pretty checked out and dismissive about the show by that point, personally.
I also find that I rarely agree when people argue that despite a season being bad, THIS episode is still amazing or THAT episode is still amazing. For me, even though there might be standouts relative to the rest of an underwhelming season, I very rarely find the ceiling of them to be as good as people say. There are late episodes that give you SOMETHING nice for a change, but still not enough, in my opinion. Even the "good" ones feels a pretty off to me.
But we don't have to agree on this.
