I'm less likely to trust your judgment, because you just challenged mine.
I was the victim of a business to business sales call. After the introductions, the CEO of the company pitching me started badmouthing a firm I've worked with. I had just finished talking about how much I liked working with them and how I respected what they were trying to do.
As she and a few other people chimed in with their take on how misguided, lousy and doomed this company was, I couldn't help but notice myself thinking less of my hosts. The only other choice I had was to think less of me… and it was easier and more fun to think less of them instead.
Far more effective, I think, to congratulate the judgment of your prospect based on the information they had at the time, or the goals they had at the time or the resources they had at the time. In fact, it's almost certainly true that given the information, goals and resources they had when they made the decision, they did exactly the right thing.
Then, because things change, it's totally okay to make a new decision based on new information, goals and resources.
Tell me about how things changed. Don't tell me I was an idiot.