“I wonder what the impact would be if you did nothing?”
If delivered in the right way, this can be a very disarming question to help build a business-case around the opportunity cost of your buyer doing nothing about the issue they have identified.
It is built on the assumption that there is an opportunity cost of inaction, and without it being a direct question will result in your prospect at least asking themselves this key question.
You are not pushing your solution, but instead are focusing on the cost of no action, regardless of who your buyer chooses to work with.
Plant the seed and see what happens.