Payment is easy if the publisher has a machine online to accept micropayments through something like the lightning network. Maybe there's a way you could make micropayments work through an email-style system or through a delegation system.
I don't think these problems are 10 year problems, I think a good team with a determined attitude could design it in one or two years (coding aside).
Making a direct YouTube replacement is probably not the best use of time though. Instead improving the building blocks or creating a system where at least the data itself is decentralized is probably better. The decentralized replacement for YouTube likely won't be a clone, just as YouTube is not really a clone of traditional TV, yet still has replaced it in many ways.
Is something like Patreon tied to youtube? Could you fund someone on vimeo or your own website with it?
We used to have web rings. We used to follow people that posted interesting stuff and weren't too annoying, either manually or by rss.
No reason it can't be done afain I think.
How would they get paid by a decentralized system?
By editing in a sponsor message in the film? By adding a few pngs with links to sponsors?
There's also the fact that people spend a lot of time, effort, and money making these videos. How would they get paid by a decentralized system?
I want to decentralize too, absolutely. But I'm at a loss as to how we'd accomplish it in a way that ultimately benefits society instead of making an inferior product.