I think your own logic is flawed slightly, if their audience is teens then surely that means they're not likely to die off quickly because these people are going to grow up still watching them all the time they're making videos? Compared to if it was 20 year olds watching them who would lose interest a lot faster as they grow older??????
Considering the age of vloggers is approx 20-25 I'm quietly confident that these aren't going to die out that quickly considering they've already made huge successes such as Zoella, PointlessBlog, Tyler Oakley etc. Who earn millions per year by making these videos. Such as Zoella's recent purchase of a £1m home in Brighton.