On a particular post, downvotes may not be able to do much but to the extent that the entire balance of voting behavior on the platform can be changed with downvotes (cough, bidbots, cough), then curation rewards to earlier voters go away too.

I suspect not as much as we would hope for. Suppose you front run a post that was originally a $50 post that got downvoted to $30 and your vote is worth $0.02 and near-optimally timed. Your curation rewards will still be quite good.