It's not quite a monopoly. Small-time online personalities can choose at least between YouTube and Twitch right now for video. I expect there are a few less popular platforms, too. Anyone care to comment on which ones?
TikTok is blowing up, I wouldn't be surprised when they grow to become a serious yt competitor.
One big difference is you can make money directly from YouTube and Twitch if you have many followers through ad revenue sharing. On those other platforms, you have to get more creative in monetizing your popularity, e.g. striking a deal with advertisers directly to do sponsored posts.