An idea can also reduce value. Or prevent you from producing value in the future. Knowing when an idea is bad or not worth doing is a skill in itself.

Exactly. "Doing it this way would prevent us from also doing X in the future; are we sure to permanently cross X from the roadmap, and are stakeholders aware?" is a valid and valuable objection. EDIT: and it can be trivially shot down by somebody with enough authority to define a roadmap saying, "we don't care about doing X".