Umm, If the Actors are doing there Jobs right, then it won't matter if they are still "relevent"
Personally, If the show only works, because people know who George Wendt is, then the show is worthless.
Since a Number of people here have talked about how good the show is, I can only assume that it is good, and therefore WHO the actors are is completely irrelevent.
Ironically this is the EXACT problem with the current Universe of Energy. Ellen is already largly irrelevent and Bill Nye while he probably has a longer shell life isn't much better.
ELLEN's energy Adventure will die, because nobody remembers who Ellen is.