You know just because OST and OKW don't have a choice of getting their weapons early that doesn't mean it shouldn't counted as investment.
It's only 15 fuel in fact most of the allies side tech(what you call investment) is super cheap.
The reason why you have to spend fuel to get snares is because non of the allied faction need to build tech to get mainlines.
OST/OKW can't get just spend 15 fuel and get LMG's/stgs or rifle nades/incendiary nades but USF and UKF can.
Just because allied factions have the choice and ost/okw don't doesn't mean that ost and okw for infantry.
And before you say you tech up also with those i'll say allied tech is less expensive. OST can't even reach the final tech level before every other faction can.
Its hard to caculate the right amount of cost from an entiere tier to the unlocks of lmg or snare since not every unit in the tier is as strong/valuable as the other. A lot of people believe no tech of any kind should ne added because of this. To my knowlidge cons are the only unit to wich it only effects them, making it easier to do the math.
And people need to stop with the cheap cons (wich they arent) beat more expensive voljs/grens and look at the bigger picture instead of those units in a vacuum.
Its true allies and okw dont need to tech to build main lines. But allies dont have units as strong as pgrens obers stock to back them up nor mg's as early or powerfull depending on the faction.
Ost has the earliest and strong lv fase as well punishing not teching at nades or zooks potentialy.
Ost fields their medium before soviets get their first t34. So them reaching their final tech with even better units later is more a luxery problem then anything else.