Replaced also carries the implication that the 105mm was found to be a superior gun, and so the 75mm versions were either phased out of service, or had production cancled in favor of the 105mm armed version.
Really, it can be taken to mean either, but here, Skorpion took it to mean that you thought the 105mm armed Sturmgeschütz replaced production of the 75mm armed Sturmgeschütz.
Either way +1. But if a proof of concept vehicle is needed beyond the M3 75mm GMC, we could get the SdKfz 251/22. Just an SdKfz 251 with a PaK 40 mounted on top, carried 27 rounds IIRC. Only reason this would be a better demonstration that a limited-traverse AFV could work would be that it has a more powerful anti-tank cannon, capable of effectivly engaging any tank in the game, and its speed is closer to that of the German Jagdpanzers, or Hunting-tanks.