Than why can't UEF destroyer have proper torpedoes than? The problem with UEF t2 navy is not the Cooper, but the Valiant.
Give Valiant (let's say) 120-150 DPS for torpedoes, but remove torpedo defence from it entirely. One on one UEF destroyer would match Sera destroyer underwater. Meanwhile UEF destroyer wouldn't break through torp defence of 2 t2 subs meaning you'd have to rely on adding Coopers into mix not only for shooting more torps to break torp defence, but also for defending from torps.
I wanted to use this concept for Nomad t2 navy, but sadly won't happen.
Honestly torp-torp defence balance never took a good look by balance team for years. There were cases with some units shooting torp defence in opposite direction for years and nobody noticed that. Torps, depth charges, torp defence need to be rebalanced from scratch.