It’s more about the general linearity of effect so we are in agreement and it’s a minor detail. Whichever way you want to present it to make it more understandable for yourself is fine. B > A is all that matters.
the difference is just that when you only use additive stuff you are less likely to make a mistake and bloat the dmg inwanted.
The problem with both systems is that it’s all driven by exclusivity and excess. Having 70 additive bonuses stacking up is going to keep the bloat working. Having 50 of those bonuses attach only to B is going to keep the gap between the two very alive. The nature of the bonuses and their numerical interactions is not problematic.
For example if A and B both had multipliers, and no additive elements, but also only had exactly 5 adjusting multipliers each with trade-offs anyone can do that in their head. It’s not a problem. It’s when you have 50 that issues arise.
Of course I did not. That thing that we call “The Meta” is the player rejection of the idea that using them both makes sense incarnate. It’s not that both skills cannot be used but instead that the player will naturally take the path of least resistance. So no matter how you conjure it if A is more useful than B the usefulness of B is effectively zero. It may as well not exist.