> One might argue that having tons of popular entertainers who are frustrated with the platform, and there being no exclusivity agreements with those entertainers (AFAIK, and contrary to the way Twitch treats its top streamers) would create an opportunity for competitors to pop up.

I wish, but it's not that easy. A potential competitor needs to not only start off with the ability to handle all the video uploading and delivery, but it has to also provide the audience and monetization. If YouTube was strictly a content hosting service there wouldn't be much of an obstacle in this regard, but it's also the discovery platform that audience members go to in search of content. The network effect is too strong because hosting and discovery are bundled into one.