As anyone involved on the major torrent communities knows, at least some people are willing to share the great artistic achievements of mankind, worthwhile documentary material, etc. You will usually find seeders for the great canon of films, ebooks, all manner of musical genres, etc. So, I assume that for videos that really matter to some loyal audience, at least some people will set aside storage space for that.

However, most of Google’s petabytes of video are inane "cat videos", people’s personal uploads that only ever attract one or two of their friends to look, or shills, and would it really be such a shame if their content couldn’t be hosted somewhere?

Communities would host the stuff that's valuable to them. That's a lot more manageable than trying to host everything on YouTube, and would just lead to instances dedicated to various topics (games, TV, music, sports, etc). With federation, it won't matter that these videos are all hosted across dozens or hundreds/thousands of instances.

Much like email, if ActivityPub becomes prevalent one day, there will be 2-3 main players hosting 80% of the traffic, and then the small independent players on the side. So Youtube will be a ActivityPub-compliant platform.

You could build a peer-tube compatible YouTube-like website. With control over the server, you could collect user data and probably stream ads over peer-tube. So there is possibly a profit incentive to hosting content.

I would totally host it if I got some money out of it. Maybe get paid for ads served through my computer.