In a fight with a limited amount of Tank Busters, eliminating multiple of them across a fight using whm is a balance issue, it makes WHM super strong, if you were running WAR DRK WHM, even in fights with frequent tank busters you could just invalidate most of them which again is a balance issue. You say if its just a problem for savage then its not a problem, but that IS a problem, classes ARE balanced around savage and not other content types, because savage/ultimate is the hardest form of content.
Changing it to do potency damage makes it really weak going from fight to fight, fights with little autoattacks, whms damage potential goes down, and fights with more they end up doing way more, its why warriors vengence has such little potency attatched and is basically just flavour.
Also as Yusikah points out, you have a tonne of oGCDs in here with very few weaving tools, and if youre playing optimally you'd have to do all of them basically on CD for damage.