I watched a couple random episodes of the finale season, season twenty, of "Gunsmoke" a few days ago and I was struck just by how excellent they were.
Sure each season of the series, especially later seasons, had very formulaic, by-the-numbers plots/writing, but even twenty years later (not even counting the earlier radio series), it was driving strong with extremely well done episodes. And everything was getting better, better cinemaphotagraphy, picture quality, scoring, dramatics -- I think if the show had been forced down to just 15 or so episodes each year, with no fillers, it could have kept going -- not forever, but at least give the series a proper ending. It ended in a regular episodes (and the less said about the bad TV movies, the better).
I think in some cases it's not that a show runs too long, and people have gotten tired of it, it's just that studios would order too many episodes, force turn-arounds in ridicuously short intervals, and show runners never tried to bring things along, keep up the good work.
Just a random thought.