Slippery slope fallacy has no place in balance discussion. But to humor you the reason OKW has 5 levels of vet is to reflect the late war German army and to make the faction more distinct.
Allies AI abilities are no more or less effective based on the vet level of the unit it's shooting at.
Allies have lots of AI in their ARMOR>>>>HE leads to squad whipes!>>>>>5 levels of vet is needed to make up for squad whipes!>>>>>Grens dont have 5 levels of vet THEY SHOULD!
That is slippery slope.
You are assuming something must be the case because a previous item was true. When in fact 5 levels of Vet are not and never have been "Needed".
I think 5 levels of Vet should stay as it makes OKW unique. But it should provide utility not direct superior combat power. Especially when the units they go on often times are elite at vet 0.