Those are good points. I'd like to add... Google may see it as their moral obligation to save everything. It's history recorded.Of course they do. Why would there be such a tight upload size limit (rather than merely times, which iirc can be deduced from header information) otherwise? Sure, it doubles or triples (or quadruples) their storage liability, but I'd be willing to bet they have originals for everything. Such a thing might be subpoenaed anyway for forensic evidence in the event of a copyright lawsuit (to show origin). There are probably more reasons to keep the originals than that (eventual upscaling? bughunting?).
This is not to say that all YouTube uploads have been retained. I doubt that. But I'd be surprised if they didn't start retaining them rather early in their history.