Odds of Hanging On
Once you've made a few high quality videos and have seen a good percentage of repeat viewers following your channel, you're probably going to be fine. At least for awhile.
YouTube is relatively new, so we don't really know what's going to happen to the current kings and queens of YouTube celebrity. When actors and actresses get old, there are still a lot of parts for them to play. But what happens when a teenage girl gets popular for her videos about school, and then winds up a middle-aged waitress at Denny's? It's hard to say.