YouTube wouldn't be what it is if they kept wondering how they'd monetize free video on the web. First they experiment, then they expand, then they take over. Even if money does not follow directly, it will eventually make its way into Google's core product: Adsense.
Youtube is pushing out of its niche and wants to become the place for storing and sharing any kind of video on the internet.
Note that Google has storage to burn, they've mastered high availability clustered network storage at a cost per gb far lower than the rest of the industry, they might as well try to use that for leverage. Also, google is a backbone internet provider, they don't pay for bandwith the same way that Microsoft or Apple might. Youtube bandwitch may cost them nothing, and even in a worst case scenario it only costs them rock bottom wholesale rates. Again, they're way ahead of the competition, so it makes sense for them to push.
Unless the idea is that when people have 4k displays, YouTube will have a tiny expensive back catalog.