It really makes you realize how many people there are in this world where a youtube personality can have millions of subscribers and yet you've never heard of them. I can't help but respect the hustle though, especially the ones that identified a niche market and cornered it. My suggestion for anyone looking to get into the game is to understand what I just wrote and think about a very specific topic or aspect of a hobby and run with it. If even just a tiny fraction of online users dig it, you can make some money.
I check out one channel where all the guy does is review vaping/e-cigarette products, and he got over 300K subscribers. Funny thing is that he would purchase the products himself when he started, and now the manufacturers just send him everything for free hoping for a review so he doesn't even have to spend money anymore. I'm talking even $500 to $1000 devices...for free...
Just think outside the box, and choose a unique topic that you really know a lot about (because people can spot a fraud or casual).. you can start a channel that only discusses 80's Saturday morning cartoons and get like 100K subscribers...