Both channels are pretty disposable to be honest. They at least seem to have more events on Teen Nick than on Nicktoons, but both of them are used primarily as channels to toss in old reruns or shows that they don't want to air on the main channel. I doubt that they'd get rid of both channels, but I don't think that Nickelodeon would suffer drastically without them either.
The only problem with getting rid of either TeenNick or Nicktoons is that would mean that the older shows would have no place to air. Even though there is another option (either buy the TV shows on DVDs or Blu Rays or watch them through online streaming), there might be some people who still want to watch the shows on TV or there might be some shows that haven't gotten a DVD release or aren't available online and being on TV is the only place they got at the moment. So, I think that one of them should stay up and it all depends on how creative Nickelodeon can get with the schedules to keep the viewers interested.