That change would do nothing to address the actual problem with Guardsman, which is that if you don't kill all the Guardsman outright (ie none of them survive wounded) then you have to kill every unit in the stack in order to get to the "weak" units.
For example, imagine a stack with a Guardsman unit, 10 units of the same type with the same promotions (except for Guardsman, which they do not have), and an Adept. A unit with Marksman cannot target the Adept because of the unit with Guardsman. If the Marksman attacks and kills the Guardsman then another Marksman can now attack the Adept. However, if the Marksman attacks and the Guardsman wins but is injured then a second Marksman cannot attack the Adept - nor can it attack the injured Guardsman! Once this happens, the other 10 units in the stack must be killed or injured (to a degree that depends on the relative strength of the unit in question as compared to the Adept) before the Adept can be attacked.
Under this particular circumstance, which is fairly common in my experience, the Guardsman promotion is effectively allowing a potentially unlimited number of other units to act as if they also had the Guardsman promotion. It doesn't matter how easy or hard it is to get Guardsman, the problem is the same: Guardsman shouldn't work like that.