I don’t think a soft vs hard no has much to do with respect (and I’m not sure why you think respectful working relationships aren’t possible). A hard no is just

I don’t think a soft vs hard no has much to do with respect (and I’m not sure why you think respectful working relationships aren’t possible). A hard no is just a definitive answer, and can be delivered as respectfully as you like. A soft no is usually an “I would if not for...”. Which some people will simply read as an invitation to remove blockers, or correct your mistaken prioritization decisions, so that you can deal with the more important issue of what they want you to do.