This is a great question. The case of a “trial” user that doesn’t convert is a useful case study itself. I would ask them (1) how they heard about the product (2) why they signed up for the trial (3) what they did during the trial that convinced them not to continue (4) what they would be willing to pay

I would not necessarily extend trials just to do that, unless it turns out from your interviews and other data that the trial period is too short. Or if you think more usage will turn them around and convert them.