OKW being able to build them so they float less MP would make OKW less OP, not more. Or do you think the current giant OKW infantry hordes are balanced right now, honestly?
What makes you think adding caches in any way is not a buff to OKW? Why would people even build caches if it was not better than just floating all the manpower? It will let them get tech and vehicles out quicker, at the cost of really nothing. The floated manpower is wasted now as is, letting OKW convert more manpower into greater field presence in no way fixes the problem.