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.

A human can use that account first to get beyond two votes, and then subsequently pass that off to a spam bot

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.

AFAIK, most captcha breakers aren't hired. They are simply bribed with porn and similar things:

"To see naked pictures of Marissa Mayer, please type the words below into the box."

Honestly, Marissa Mayer would appeal only to geeks & nerds. And those might be more interested on what she can do, than look.