I honestly think Nick should can SB after this season. The original creative fire and fanfare it used to have simply isn't there anymore. They shouldn't let SB become another Rugrats (then again SB hasn't even come close to sinking as low as Dil-era Rugrats yet). After cancelling the show they'll get 2 or 3 (or even more) years of reruns and an almost guaranteed spot on Nicktoons Network probably forever.
No matter whats happens though SpongeBob is gonna be immortal on the greatness of those first 3 seasons alone, just like Rugrats is still revered for it's pre-movie run. I wouldn't worry too much about it's future, the writers and creators are gonna be living comfortably for a while now just on that.
