There are some absolute lunatics posting their content out there, but that will always be the case. I don't think it's a problem that really needs solving. Even if it was, I don't think that this is a solution. If you are too quick to counter a statement, people may suspect that you have ulterior motives. If videos come along with these "corrections" many viewers will see that as proof that they're right and that the "mainstream" is trying to cover something up.
A video is a creative production. The creator makes it in a certain way for a certain reason. When the publisher interjects, tries to mold and reshape the content, provides their own commentary with it, that restricts the creator's artistic freedom over their content.
I don't expect that this pseudo-censorship (that's what it is - they're saying "we don't think your content is right, so we'll be giving your viewers what we think is right) will be applied neutrally, either. There are nutcases in all ideological directions, but I have a solid suspicion that these annotations will only be applied to those that are not in line with the "politically correct" zeitgeist.
Of course, YouTube is well within their rights to do whatever they want with their platform. This isn't at all an uncharacteristic development for them, either. They've consistently demonstrated that their only interest is money; they don't care for the small creators - or really even the viewers - all that much. If this is what keeps the advertisers happy then it's only the logical step for them.
I just don't like it.