Shows overstaying their welcome has always been such a massive issue. Execs see how popular a show becomes and demand more ASAP, even if the show was never meant for it. It's pretty old now, but Heroes is such a good example. The writers wanted the show to follow an anthology format, mostly new characters and stories every season, but seeing the cult following, NBC wanted more of the same, despite the writing not being there to support it and it went downhill fast.