> Your largest users eventually stop posting if you don't pay them (because they can go elsewhere after using your platform as a springboard: see Vine)

I don't think that's as big as a problem as you do, as long as you don't care about exclusivity.

Think of the streaming music market: Youtube Music, Spotify, Tidal, Amazon Music, ect, generally have mostly the same content and little exclusivity.

For example, you could have a feature where all uploaded videos are automatically uploaded to YouTube and all of your competitors.