I don't think there is downvote option as such, only flagging a post which is a bit different. I kind of partly agree with your suggestion, but then again there are a lot scenarios (like now) where I only feel like commenting, but not voting (because I don't fully agree). My two cents is that you should put more thought and research into this thing, and if you still think it would benefit the platform, then rewrite the suggestion on stronger grounds.