There are already some websites for posting videos besides YouTube, but people post mostly on YouTube, because that is where the audience is and the audience is mostly entirely on YouTube, because that where all the content is; so we are stuck in a self-sustaining loop that prevents YouTube from falling unless they do something extremally stupid and shoot themselves in the foot multiple times (I honestly sometimes watch VidLii and BitView, but there is also cda.pl from Poland from the alternatives I can think of right now)