This is bad comparison tbh. Doctines dont, in a global scale, break faction rules, they allow different playstyle with-in the faction rules.
When they are truly breaking the core of the faction we have VSL, pre-nerfed SVTs and osttroopens. Its been proven a lot of the times by now, that its very dangerious to add tools which potentually just strateforward changing how faction works.
Yet we still have all of those after they have been adjusted. VSL in particular have been in the game for a long time before they became meta, because at the time the Tiger - notably, a unit with which Ostheer was designed - overshadowed them heavily. The doctrines I mentioned all break the "rules" that the core of the faction indicates. We could even add more: UKF and USF getting long range artillery, UKF getting a mobile mortar and Ostheer having the best long range TD in the whole game. My point is that this "design" of filling holes in the line up with doctrines is not a new concept. I would even argue it was supposed to be like that from the get go, at least for the eastern front armies (especially considering Soviets getting elite infantry via doctrines while having only trash infantry in their main tech paths).
This is not a new concept and we find it in a multitude of commanders, many of those are considered to be fine, some UP, some OP. If it is a balance issue, then not necessarily because this hole has been filled, but more often than not because the unit itself is not properly balanced.
I agree though that yes, it can cause issues. But I don't see how in the case of a "mini Panther" EZ8. If USF cannot have heavier armor than 160 there should be no Pershing, o dozer upgrade and no EZ8 in the first place. Pershing and EZ8 are barely used in the game. And on top of that, Axis can deal with heavy armor as proven with the existance of Soviet doctrinal tanks and UKF tanks.
So if a) USF already can field some heavier armor and b) Axis can field proper counters, I don't see a reason to cancel a "mini Panther" EZ8 already at the theoretical stage as OP.