As many probably realize, the fact that "online videos" == Youtube for many members of the general public means that it is by far the number one source for both

As many probably realize, the fact that "online videos" == Youtube for many members of the general public means that it is by far the number one source for both posting and watching videos online in a self-reinforcing loop. I think that it cannot be overstated how much the existence of platforms like that (e.g. video sharing websites, 2D art websites, music boards) drive non-professional creators to create, given the fact that for the first time, they can actually get an audience. So it is not so much that Youtube and similar platforms are necessary for content creation (which is a ridiculous claim that no one made), but that the absence of such platforms seems likely to result in the previous system where media was very rarely shared by the non-professionals. If you are arguing that massive content consolidation platforms such as Youtube do not have a highly significant impact on content creators as a group though, then I do not know what to say to that.