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That is exactly the reason why you can. T-34 has same price tag as stug but more pop. Why would that be considered right?...
For a a very simple reason one is main battle tank able to fight both soft and hard targets and the other is specialized unit with an AT role only (ok it has mg).
Specialized units are designed to be cheaper and cost less pop for their performance.
Think about it in another way, if a unit like ostwind was available in the Soviet T4 there would very little reason to built it (accept for AA) over the T-34/76 since they cost about the same, have the same pop and although Ostwind is a bit better in AI T-34/76 is a far better choice.
If a Ostheer player spams stugs and invest all his fuel in them he will probably find himself in trouble because he will have very little AI.