There are other potentially useful differences, too. It operates on a per-post basis, so could be used with more granularity, and it could potentially help to normalize the downvote in the ecosystem. Along with that, maybe "downvote" could be renamed to something like "deboost" or "downrank".

But, yeah, I don't know if the potential for retaliation is surmountable. It's an interesting idea, but hard to guess how it would play out in the real world.