Let's say a site has plenty of users. Someone asks a Meta question and someone answers it with a proposal. Maybe 90% of the users dislike this proposal, but without down-votes, the proposal can only gather up-votes over time, until it appears to be so popular that it is considered community consensus.
This could be a compelling argument to enable down-votes at least on meta — but there is still an alternative:
Simply propose the opposite and let that compete on up-votes. Something along those lines happened here:
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I'm not sure that this is quite a "duplicate", but there is already a healthy discussion about the desirability of having some disapproval mechanism (a.k.a. "DV") on the site:
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OP's scenario here is another one to consider, apart from regular "Q&A" which is the focus of the linked discussion.