I may be wrong, but if each poster had an IPFS server with their videos, then videos would not disappear.
This was a good video:
An idea that I think would work to keep videos avaliable is thumbups would store the video on the users server. IE I up vote this video, A copy is then stored on my PC and be available. If this was implemented, this video would be distributed on 185 computers.
https://steemit.com/izlude/@izlude/preventing-your-dtube-vids-from-disappearing-aka-hosting-a-node