The proportionality doctrine is fine enough in theory, however in practice it leads to escalation. It may prevent large leaps in escalation in the short term, but repeated acts of aggression from each side will slowly raise the stakes. What if an air raid on munitions factories accidentally level a neighborhood, killing civilians? Would the proportionality doctrine allow for retaliation in the form of more civilian casualties?
The proportionality doctrine allows for mistakes and accidents to turn into "justified" war crimes for each side, which are unethical.