Well...I can't really agree with that for the simple reason that there may be hundreds or thousands of other proposals (or even more than thousands). Theoretically, yes, you can vote for all of them without worrying about a manabar and this is equivalent to a downvote, but in practice, no not really (also one might even consider RCs at some point).

But the structure of having return/burn proposals is very much like downvotes in practice and it seems to have worked well for Bitshares so I say we should give that solid chance to work before jumping to downvotes.