No one’s stopping you from disagreeing with someone’s viewpoint. You just have to do it as a comment, not a thumbs-down. Many GAs/GQs aren’t given meaning “I agree”, they’re given meaning “well written” or “You expressed your viewpoint beautifully, and I understand that side better now” or “You really made me think”. For questions that are less opinion-based, like “will drinking water help cure my hiccups”, a GA can mean “I agree, what this user said”. But if you disagree, it’s helpful not to simply thumbs-down them, but to say “no, drinking water is actually dangerous when you have hiccups, because according to the MayoClinic, it causes blank blank and blank” or “You can, but it doesn’t do anything one way or another”.
Besides, many jellies came here from sites that had thumbs-down and didn’t care for that feature. But if you really want it, I’m sure many a jelly could refer you to an Q&A site with that feature.