I didn't mention writers at any point. The writers for a show are mostly filling out the creator's idea, but ultimately have little say in the actual direction of the story, or how long it goes on. Guillermo del Toro is ending The Strain after only four seasons, because his story is done. Kurt Sutter ended Sons of Anarchy after seven, because the Sons' story was done. He's working on a spinoff about the Mayans MC because he still has stories for them, not because FX sat him down and demanded more. SoA was over, and Kurt went to them with his new idea because there was no way they'd turn him down.
As for shows that overstayed their welcome, Lost is an excellent example. That story wasn't long and needlessly convoluted because the creators were so clever and deep, it went on because people inexplicably liked that drivel, and the network kept drawing things out. Another great example is pretty much any sitcom. Although those are mostly garbage from the start, sitcoms go on for a decade or more (seriously, can The Simpsons just fuck off, already? It hasn't been good in 20 years), and only keep going because they appeal to the lowest common denominator. Friends, The Big Bang Theory, Two and a Half Men (they fired their star and still kept that pile of shit going for another few years), etc.
If the stars of a show are renegotiating their contracts and doubling their pay every other year, it's not because they want a fair share of the money the show's making, it's because they don't want to keep playing the same role they've spent ten years playing, and the network execs are piling money on top of them because those paychecks pale in comparison to what's going into their own pockets.