Currently you can build the VI B as soon as you have built all three buildings, regardless of whether or not they get destroyed. This needs to changes as it is impossible to prevent OKW from building the buildings (due to cheap teching costs) in any remotely close game. Furthermore, OKW can easily float with volk schrecks/obers (particularly with a forward t1/t3) until they shit out a VI B. This is very similar to the tiger ace in that you can punish someone for poor play and then have them magic out a very heavy unit.
The VI B should require all 3 buildings to be currently up in order to build.
Imagine losing your Major, and therefore you wouldn't be able to get any T3 units - that's what's happening to OKW. Losing any tier building is a huge step back, and not just some minor loss that can be easily replaced.
They get T1 / T2 for "free", but pay extra for any unit or tech. Their fuel starvation obviously also affects teching, meaning in a balanced game when OKW just pops their 80 fuel building, you already gathered 120 fuel. Now add the 3rd building, and KT costs, and you would gain approx 570 fuel till the KT arrives, while facing 0 tanks in between.
The KT is more of a last ditch unit, and not something you'll see every game, nor is it that useful in general.
just my 2 cents.