Lets narrow the question a little bit. We the viewing audience can handle as many quality seasons as a show can give us.
The problem is shows, and we're really talking specifically about high concept genre shows, need two important things to even make it onto TV. They need a concept/gimmick/premise that can get the show picked up. And they need a producing/writing staff that can sustain that premise.
For most genre shows one or both of those two things does wear out after a couple seasons. And I think that is due to the sort of showrunners and writers that are attracted to writing the sort of stuff we like to watch. They are extremely creative people, but it is also hard for them to stick with the same premise and keep plugging away at it day after day, season after season.
JJ Abrams creates some of the most high concept TV shows around, and almost immediately abandons them to lesser producers. Ron Moore seemingly loses interest in any show he writes for after two seasons. Joss Whedon could have stayed focused on Buffy, but got sidetracked onto other projects.
All of our genre writing heroes have an attention span of just a couple years, before any given show or project is stale to them. And once those creative forces are no longer fully committed to those projects they invariably suffer.
Yeah, budgets, ratings, all of that stuff matters as well. But we're never going to get a really good long lasting, sustained high quality genre show until we get some showrunners with a stronger sense of loyalty to their creations.