YT probably does need to exist for many creators to get the kind of distribution that enables them to monetize their content including the operationally managing the scale of the site, the brand they've built and the ad network. A highly viewed piece of content is generally much more valuable than unseen content. Which isn't to say you can't monetize video outside of YT, just the YT is providing a very valuable service and it isn't clear to me that is qualitatively of less value than what Roblox provides its content creators.