I agree with you in all of this. But here is the thing will you say it's hard to make a non doc option viable when it's non doc version is "not weaker".
Not sure if I get this right. Do you wanted to say it is hard to make a doctrinal ability viable when the stock option is already similarly good?
Well yes, that is the point. You could just go with the standard build. But if you want to pull off a certain strat, surprise your opponent etc, these commander abilities might be more useful than the stock options that you have.
In my eyes this is a way better option than making it OP by design and trying to fix the hurricane of problems that will spawn from it.
And we have a lot of working examples. Panzerfusiliers and Volks is probably the best one, but others and I have given plenty of more already.
P.S. don't put too much mind on the KT and t34 thing, it was(well supposed to be) an analogy on if i have to pick x among x, y, z to do A i might as well pick the strongest.
And I'll counter 1 thing, T3 ins't used because it's more cost efficient, you'd simply loose due to not having any options before you get that far(t4).
The exact units don't matter. Fact is - as I already mentioned - NONE of the generalist heavies are used much anymore. So why, if this game promotes "strong" units over cost efficient ones as much as you say, are they not built? Why do all of these other examples that I gave you exist?
And regarding your T3: I was talking about T3 spam. Yes, you can play 1 P4 into T4. It is also viable though to play a 45 min game in T3 only and spam P4s all game long. This should not exist as well if they were so cost inefficient. Also the StuG should not exist because Ostheer already has a heavy TD. Still the StuG is a very good option because it is simply more cost efficient vs mediums.
May I ask though which games modes you play?
It just came to my mind that this might be a reason. If you play primarily 3v3+, heavier units are naturally favoured because of higher resource income (reduced opportunity cost for skipping weaker units, putting on early pressure is less rewarded in general) and higher abundance of AT. It is quite frequent to eat 3 shots in the late game as alpha damage. Weak units can't sustain that, while e.g. a Comet and Panther can make it out alive more often and therefore burn less resources overall.
Smaller modes do not have this issue as much, so lighter units are more viable. if you play mostly 3v3+, then this might obviously bias the opinion (just as much as playing only 1v1 does etc).
However the game is mostly balanced towards 1v1 and 2v2, since units are way more predictable in these environments.