I don't agree. In the bigger picture the StuG is there to handle mediums that the Panzer IV can't handle (Sherman, Sherman premiums, T-34-85) or it's a way to out-resourcemanage your oponent by nullifying/destroying a more expensive vehicle with a cheaper vehicle (or just to save your ass when you're behind on resources). At least it will be after the patch, when it's a bit more reliable versus mediums (buffed traverse will stop it from being run around so easily).
I think the StuG/Panther interplay is balanced well for medium tanks.
The StuG will do a little better than the Panzer IV against mediums for 30 FU less. It can also tackle premium mediums that beat the Panzer IV, like the upgunned Shermans and the T-34/85. Plus it's cheap: you can get one out fast when you need it.
The Panther has the damage output of one StuG for the cost of two, but it's slathered in armour, has twice the health, has weak but not negligible AI performance and
very importantly has a turret.
A Panther does to mediums what a Panzer IV does to T-70s. Two StuGs put out more damage, but they can't hunt tanks the way a Panther can.
There are benefits to both. The StuG holds its own against more expensive units. The Panther
butchers anything cheaper than it that isn't a dedicated tank destroyer, and even then it has the edge within 50 range.
Then the heavies rock up and, as usual, ruin everything.
The StuGs do approximately fuck all to the heavy: they can't penetrate it from the front and as casemates they can't flank it. I've thrown three of the things at one once and watched them struggle to do half damage at point blank before they all get killed.
The Panther can penetrate heavies from the front
and flank them if it wants.
Unless heavies are off the table, why would you ever risk StuGs?