In the proposed scheme you can't upvote unless someone upvoted you first. So you can't create a second account to upvote the first, because that second account needs its own upvote before it's usable.
Yes, but that would mean that he took the effort and used his skill to present a truly interesting comment. This isn't particularly hard, but I think harder than a bot or hired non-english speaking captcha breaker can perform.
We don't have to make an unbreakable system, just one that makes us less profitable than the rest of the internet.
