Because you can “overcome this with content design” without building it on top of the shaky foundation of overpowered tank CD’s that necessitate a hard floor for the tanks and make balancing incredibly messy
If BW is worth an arbitrary number (let’s say 100) and the damage to the tank is say 50, you don’t up the damage to 150 then make every tank have 100 as a baseline because it both makes it harder to balance and ensures that if the tank can’t press that 100 button then the entire thing becomes unclearable while also providing no benefits
Reducing tank sustain to a 25 and leaving damage at a 50 is way more viable, future proofed and easier to balance than trying to balance of the foundation of assuming every tank starts with a 100