youtube probably doesn't save the originals (though they could in some cold-storage tape drives, perhaps). But even still, it's not difficult to imagine that there may at some point exist a compression algorithm that can be applied to existing compressed video that could change a couple bits around in whatever encoding scheme you've chosen. Depending on the file type, that could be enough to corrupt the whole thing.

Sure you can get around this by adding ECC, but that isn't implemented here.