So what do you expect, after a plunderer fights your support and wins, let's say the support you have has higher power than the plunderer, you expect him to still be able to win your team which is just a little bit below his own power?

Right now you lose 13 coupons only, but the plunderer would lose 50 coupons every time if this feature changes like you said. If you're smart you would chose the strongest support, or do your convoy at unexpected times, or both.