It depends on how this show will turn out and what events may happen with this in the future. Minus a few poorly written episodes of this show here and there there's still a lot of spongeBob fans and a lot will always love this show. Heck even I still love the show and I am not gonna follow the other teeanger's footsteps and not be affected by any possible peer pressure unlike some others have. This show shouldn't be a show where you think there's an age limit, and once you hit that limit then you are too old for the show. Nah that just doesn't seem right, this show is so great (especially that move that came out in theaters) that there should be no age limit for this show, it rocks and there's still a lot of great episodes to go around, even in the newer seasons, in my opinion one good example for a great season 4 episode would be Patrick SmartPants.
(I do agree though with some users on this board that some of the more recent episodes show the characters have gotten more childish.) But still, everyone should watch this show no matter what age they are at since you can still catch reruns of season 1-3 episodes for this show back when it had more adult humor.
The way i see it this show still has a lot of fans and the past episodes (especially in seasons 1, 2 and 3 and the movie) are still living on and SpongeBob will always be an instant awesome classic! So to me this show I'm sure will have at least a few more seasons left, since wikipedia is a crappy web site. (Never trust Wikipedia please.)
SpongeBob_SP says 60 new seasons? Well I dunno about that but it is good to think that will happen because it would be nice for this show to last for more time to come, it also would be nice if this show could run for a very long time like The Simpsons!