Earlier quoted context omitted.
There were failed social media platforms before Myspace and Facebook. If you'd have said Facebook would outgrow Myspace people would have thought you were crazy. At some point someone is going to unseat YouTube. It's going to take timing, luck, and a product that delivers something better or different than YouTube. Is this it? Who knows? Most likely not, but that's why you keep trying.
The person you are responding too isn’t saying that youtube will never be overtaken, he’s saying he doesn’t think it will be overtaken by dozens of smaller content platforms, but rather one large one.
But you don't become a large company without first starting small. Even large corporations tend to use pilot programs in order to establish product market fit. Disney for instance tested their software on less valuable material before deciding to create Disney+.