They only changed it for the videos with vulnerable URLs. Do you really think it's better for all those videos to become effectively listed?
Maybe they could have done something more clever, like only private videos that aren't linked on the public web already, but there would still need to be a mass privating of most unlisted videos.
There is no way an unlisted video should be treated as a private video.
And it won't be the last time Google changes opt-out, that's for sure.