Regrets are mostly irrelevant to this discussion. A "regret" is either:
- I now have information I did not have at the time I made a choice that binds me now.
- I changed my mind.
Whether I believe it or not makes no difference because it contains no information.
As an aside: none of this is supposed to invalidate the experience of people having regrets, or struggling with addictions. If I see a friend getting drunk every night I won't go "ah, yes, they are correctly maximizing their utility according to their own discount rate and inter-temporal budget constraints". They are separate conversations, though, and we can have the kind of conversation where I help you solve problems or the kind of conversation where I just listen to you. They don't tend to mix well.
However, you could also choose to believe their regrets, when they look back on their own choices and say that the pleasure than they gained from them did not repay the misery they suffered because of them.
If you choose to disbelieve their regrets, then maybe you would agree to label their regrets as stupid?