It’s still just as expensive, you’re just adding administrative overhead.
You’d also spread the cost to more people, true, but who would operate a server for free (based on donations, but if it’s federated why should I pay for that one server?). Also, do you trust all those people to keep operating the storage for years to come? Or are you done with losing access to videos, because someone lost interest in running their instance?
Storage and bandwidth costs for video on demand are so incredibly high, I don’t think we’ll get a federated alternative to YouTube any time soon.