The signal transmitted by downvoting is "I don't want the hear this" or in simpler language "shut up". This should be reserved to fight content which is offensive, spam, trolling, rampant crackpottery, blatant off-topic etc.

I think you're drawing a false equivalence here. While a downvote does carry the meaning of "I don't want to hear this", most of the meaning of "shut up" is connotation, not denotation, and those connotations don't necessarily carry over.

Mere disagreement generally isn't enough to justify a downvote, no. But we want to see well-reasoned disagreement: it signifies a chance to teach or to learn, even if it's unpleasant in the moment. On the other hand, there are plenty of things short of Time Cube or cat memes that one might legitimately not want to see here, even if posted in good faith; restricting the option to those most extreme cases robs it of most of its power to improve discussion.