Still - "Is that your true rejection?" should be fair game for Disagreers to humbly ask, if there's any productive way to pursue that sub-issue.

Perhaps it should, but the problem is that answering this question is one of the big problems in salesmanship: working out the customer's true obstacle to wanting to buy from you. Salesmen would love to be able to get a true answer to this question - and some even ask it directly - but people tend to receive this as manipulation: finding out their inner thoughts for purposes of getting their money. This feeling happens when selling someone on an idea as much as it does when selling an item for money.

Vladimir Stepney also notes that they may not know the answer themselves. (Salesmen are aware of this problem too.)

As such, asking the question directly - as you note in your "if" - may end up being counterproductive. If this question occurs to you, you're already in sales mode. If you really want to make that sale, this is a question you have to divine the answer to yourself.