That is flawed logic.
Same argumentation can be used about upgrades(which soviets don't have), reliable indirect fire(which UKF and USF don't have), light vehicles(which Ost don't have), forward retreat points(which ost and sov don't have) and many, many other features that X army have and Y don't and would benefit greatly from.
And lets just ignore placing shreck and pfaust on the same level in your post.
Model count isn't that important to throw all balance achieved so far out of the window.
UKF players often NOT upgrading to 5th man is the very best example of why 5 men grens aren't needed.
And the next person implying stuff I did neither write nor mean. There must be a nest or something... There is hardly any flawed logic since I did not compare Grens to Riflemen, IS or Conscripts for Christ's sake. I am comparing them to Volks. I am comparing them to units they will never play against in Automatch. Which is exactly why I am questioning why the small difference between those units does forbid giving both 5 models. There is no logical reason.
You point about Pschreck vs Pfaust is nitpicking for the sake of nitpicking, since it only proves the point about Grens I am trying to make here, if anything. Of course a Pfaust does not equal a Pschreck, thanks for clarifying. Maybe that's the reason why Ost gets PGrens later on while OKF gets no additional infantry unit with AT capabilities. Mind = blown.
Let me try a third and last time. Volks and Grens have the same jobs, their only difference is the order in which they unlock stuff. Grens start with light AT and can upgrade to decent AI, whereas Volks start with light AI and can upgrade to decent AT. Those details are all absolutely unimportant for the simple point I am making here, it's astonishing how they are still brought up just for the sake of finding something to bitch about.
And yes, model count disadvantage combined with no forward retreat option actually IS an important factor that can offset balance.
The only thing that is flawed here is your UKF comparison. Maybe the reason people don't use it much (source btw? your own games only?) is that it freaking costs something while not reducing reinforcement costs and makes the most MP starved faction in the game bleed even more. Just a distant possibility, yet worth mentioning, I think.