Well at least if you pull numbers out of your arse and then make a decision based explicitly on the assumption that they are valid, the decision is open to rational challenge by showing that the numbers are wrong when more evidence comes in. And who knows, the real numbers may be close enough to vindicate the decision.

If you just pull decisions out of your arse without reference to how they relate to evidence (even hypothetically), you are denying any method of improvement other than random trial and error. And when the real numbers become available, you still don't know anything about how good the original decision was.