We all know shows that started out strong then got ruined by this stuff, like Lost or Heros. Or the internet's favorite punching bag, Game of Thrones.
I think The Wire is an interesting exception. They never got more than a single season commitment, but still managed to make something that feels cohesive over 5 seasons. They did it by using an ensemble cast where character's arcs can be extended in later seasons, but with new characters entering the mix as well. They also picked a different theme for each season, looking at the failures of american institutions from different directions, and resolving that theme within each single season. The result is each season feels distinct and can stand alone, but they also form a cohesive whole.
Heroes, I agree with you, no question.
That said, it's also aided by primarily being a story about characters rather than plot.
I always thought Lost got a bad rap for that. It was actually a story about characters, but its fandom thought it was very much about plot. Hence the hate for the finale (which I thought was wonderful)
However, genuine thanks for the Lost finale spoiler, I never wanted to watch beyond season 2 anyway as it already became rather tedious.
https://www.youtube.com/watch?v=PdOpQE5miP4
Referring to the 'Who Shot JR' part was a mixup on my part- was pretty young at the time and not actively watching weekly so I forgot which was which...