Rubbish. Assumptions permeate everything we do in aviation. Every performance calculation and every SOP is based on certain assumptions.
If I need to descent right now! I need to do so based on a number of assumptions as I do not have enough time or enough information to do a full analysis of all the factors at play. One of those is that if I am on the airway someone might be directly below me. By and large if someone is going through my level 5 miles or less from me I do know about it if they have a transponder. It is my job to know and be aware. If I don't happen to know about the other traffic either turning or continuing straight ahead might result in TCAS as I start the ED maneuver. Since I do not which I just have to take my chances and follow our SOP which requires a turn (based on a number of assumptions of course...)