This is not an idle question. It's a critical question for informing a decision-making framework that rests on calculation. Which we need here - I think we can all agree that personal ick-factor is a bad approach to large-scale public policy.
Which is to say you're right. We cannot be certain that won't happen with this approach. Indeed, we cannot be certain of all consequences with any approach.
With that in mind, what level of certainty should we require?