I've often wondered this myself. It is the nature of all things entertainment. Vloggers can only have a few years run at best. Look at Seinfeld, 180 half hour episodes over 9 years. Look at RayWilliamJohnson, 629 five minute videos over 4 years. I don't personally watch him so I am only commenting on statistics. Throw in a couple of half hour shows with Ray and the total amount of produced content on both Seinfeld and RWJ is about 5000 minutes. The difference is things happen much faster now, a new video every three days as opposed to 26 episodes a year. There is only so much that can be done until someone runs out of steam, ideas, money, etc. If you make your channel about you then you might have an issue.
The nice thing about YouTube, at least for now, is that you can take your experience in creation and apply it to different subject matters. If you look at your interaction with youtube more as a director than talent then your skills are transferable to other genres. Take Markkaz for example, is the toy cars market drying up? He can move on to something else. He already knows what it takes to make it.
I am hopefully still on my way up. I can see at some point where the group of people that watch me will get sick of what I am doing. I've already got a few ideas in the hopper for new and completely unrelated channels.
I read that article that Just Some German Dude posted. I think it is a little twisted. It talks about some of the original YT creators and how they were just on YT for the "community experience" and how business ruined things. Sorry, business is what is making YT the new media. Without it you have just a bunch of narcissists making videos. I've said it a million times, I love making videos but my desire is not to do them for free. While it would be nice to "get rich" making videos I will settle for making a comfortable living doing something I like. I think quite a few people here just don't understand responsibility. When living in your parents basements you can do anything you'd like as you have no responsibility. When 5 people look to you for their next meal and you tell them you want to film a youtube video, it had better benefit them just as much as you. In reality YouTube is just a bunch of starving artists, akin to the waitress in Hollywood that calls herself an actress. Without business she can never become that actress, so why do it?
