1. They've lasted longer than most shows...5 seasons is a long time for a show like that.
2. Kevin Spacey isn't just "one worker", he's the face of the show; the single most important "worker" on the show. In addition to lead actor, he's also an executive producer on the show.
3. They gotta take the good with the bad. The show's success is due, in part, to the big name Spacey built over 30 years as a Hollywood actor, if it's failure comes from his pedo nature over those 30 years, so be it.
4. FTR, no one has anything against the show (morally, with respect to these allegations), so if Spacey wasn't so intertwined with the show's success and it was actually still a good show, they could write him off the show and keep it moving. But alas that's not really an option.