I agree, but the general consensus in startup pop-literature (is that a thing? :D) is that you can never be sure unless you ask money and people are willing to pay. Otherwise people can just say they like / want / need it, without risk. I have to say that I agree. Too many times I've experienced that people _say_ they'll pay, but never actually do when push comes to shove.
But when someone does agree to pay (meaning you've somewhat validated your idea) then you're left empty handed and have to backtrack with your first customer. Not a great start. But that's just me :) YMMV