Family Guy was cancelled from 2002-2005.
It ran from 1999-2002, but there were a lot of disagreements with Fox, and Fox basically changed their time and day and when viewer ratings went down (since nobody knew when it was on) they used those numbers to justify cancelling the show. (Same happened with Futurama, and both shows have moved to other channels, Family Guy to Cartoon Network, Futurama to Comedy Central, and both shows still take shots at Fox to this day)
In addition, it was the DVD sales that brought Family Guy back. It was the Comedy Central Viewership that brought Futurama back.
Futurama was cancelled from 2003-2008, but in 2008-2009 they made 4 movies rather than episodes, and they didn't resume making episodes until this past year.
My point is that I'd have cancelled South Park well before cancelling Family Guy and Futurama. Especially because I feel like the worst South Park episodes were the early ones, whereas the best Family Guy and Futurama episodes were the earlier ones.