1.) An open source alternative to YT will be able to offer an alternative source for the same content repository. People will likely start having the same video posted to their YT account and to their open-source website as well. So famous YT channels, like PBS Spacetime or Anton Petrov will start posting videos on both YT and the open-source alternative. Just like streamers stream on Twitch and YT at the same time.
2.) Monetization can also be done through a third party internet ads firms (sort of like Taboola maybe) that specializes into placing correct type of ads on videos of creators who subscribe to that service. Monetization is already bit fcuked with most content creators having to rely on sponsors anyways. But yeah, money for running data centers for holding all that user data would be a problem I have no solution to. Maybe the online company monetizing the videos can offer a higher cut to the creator and a no-demonetization policy as well. That will bump up earnings of creators to allow enough so that they can pay for video storage and still make as much money as on YT.
3.) Banning YT is simply not a viable option. YT is much more subtle in its censorship than Twitter, for example. Whatever the alternative, it should be more stable and self-sustaining than YT. Only then will it be successful.