I agree to that. The StuG E would probably be fantastic in the hands of OKW, OST already has a lot of AI options both stock and doctrinally and even at similar timing.
I get the argument that OST has comfortable lesser Panzer IV (G) that is a safer bet.
It would probably be theoretically a better option to consider in the hands of OKW, but imo end up being neglected in favour of doctrinal Ostwind or Luchs simply due to its micro intensive requirements and underwhelming performances.
If an OKW player keeps a luchs alive for 5 minutes he gets a lategame killing machine that can suppress squads in green cover.
The Ostwind is a solid tank that can hold off t-70s and even Stuarts and AEC when properly supported
The Stug E is simply unrewarding, so even OKW, with its expensive premium Panzer IV (J) would have better Tier 3.5 vehicle options.
What the StuG lacks (or lacked? Let's see what the patch changes bring) overall is a place: Both in the OST line up as well as in different game modes. The 50 range are a decent difference to the Brumm: Support gun instead of breacher. But I think it fell short on the game modes: Casemates are barely used in 1v1, and for larger modes it is still somewhat fragile. It had a window of maybe 2v2 but that might not have been enough.
The extra range doesn't really play in its favour with the slow projectile and casamate set up. As a support gun, to my experience it's inferior to even autofire 120mm mortar and leIG options
I think it would probably work better as a direct fire turretless anti infantry tank with a su-76 barrage ability.
Casamates may be less used in 1vs1, but they are definitely still present, especially Stug G, Jagdpanzer IV ( and SU 85 occasionally.
The reason they are less used imo is that heavy tanks are much less common as well, and since most (all but the stug E and Brummbar) casamates are tank destroyers this makes premium mediums like KV1, T-34/85 and Panzer IV, generalists capable of engaging all targets especially with atg support, a better investment.
I don't think it has to do with casamates in general.
Way too ineffective alone, so it needs to be used in pairs, which makes pathing super awkward.
And at that point it's just much more efficient to use a Bruumbar