It unfortunately doesn’t quite work that way. For example Destro which is currently extremely strong is solely performing due to it’s 4pc tier bonus. Without it it is a D tier dps.
If you lowered it’s spell damage you would essentially break the class for as soon as the tier set becomes irrelevant.
Doing batch balancing is honestly one of the current issues of the balance team. They buffed Destro by 5% overall prior to people having 4pc which was warranted, but they did it despite the 4pc being extremely overpowered and then did no adjustments to the set bonus.
Logic like “hey, maybe if you’re going super deep into Mythic prog you should learn more than 1 spec your class has, especially since both of those specs are S Tier on that fight” doesn’t belong here.