I got to quibble with a few of these.
So far as I can tell, you only quibbled with the Wire. My premise was, "a four-year cutoff seems no better than any other," and I don't see any real quibbling with that otherwise.
Like South Park, The Simpsons are just riding on legacy fumes. Maybe 1-2 episodes worth watching a season at this point, but hardly worth keeping up with.
But was the end of Season 4 when it started sliding? I'd argue no. If the show only existed for four seasons, it'd probably be forgotten by now, as honestly much of seasons 1&2 weren't really that good (like TNG, it took quite a while to find it's rhythm), and seasons 3 through, oh, eight or so really had vaguely the same ratio of good to bad episodes.
Most shows just don't get that length of time anymore. Is it execs learning their lesson? Of course not! Execs never learn lessons.
One of the issues is that sometimes the execs learned lessons very well. If your goal is advertising dollars in hand, keeping that zombie cash cow going is often incredibly sound policy.
Friends, Big Bang Theory, HIMYM, heck,
Bonanza and
Gunsmoke all had better ratings in their last seasons than a generic new tv show on the respective network at the time would have (not hard, I guess, since I think half+ of all new shows fail in 1 season). We all hate it (particularly if we used to like such and such an IP), but one has to remember that, especially with network TV,
advertisers are the consumer, and
we are the product, with the shows just being part of the production process (too jaded?).