> How do we get good content from a platform that is explicitly copyleft?
The same way we get good software from a ´community´ which is explicitly copyleft.
> Will the vast majority of creators use this even as a backup?
The best way to use platforms like Peertube and Odysee is by using it as a primary source, with Youtube and Vimeo essentially playing the role of CDN. Should the censors not like your video for $reasons they can block them 'till they turn ultraviolet but they can not deny anyone access to them. Make sure to advertise your alternative distribution channel(s), use the same channel names and video titles so they can be found through regular web search and you're set [1].
[1] that it, until censorship reaches the network layer
