W-e-l-l, doubtful if they ever had the budget* or the props to go full-on military anyhow. Even if GR allowed it, and I question if he would. E-VER.
As a keen armchair general, I too would like to have seen some proper military @$$-kicking.
Later series (IMO) REALLY could have used Star Fleet Marines or whatever the MECOs of Enterprise became at various stages (notably DS9, but all the others showed the need as well). Star Fleet personnel with just nice uniforms and hand-phasers beaming planetside for science and/or diplomacy was acceptable - but deliberately herding them into serious ground combat that way as well never made any dang sense. Doesn't prove moral superiority, just means stupidity.
That entire mess, I lay directly at GR's door. As I understand (and admittedly I could have misread or misremembered), he was always totally opposed to 'military concepts' (such as 'Space Marines') intruding upon his vision.
(* - Reminds me somewhat of a popular TV cop show done here in Australia during the 80's. In its first season, you would never
see more than two characters in it at a time with guns drawn - whether publicity stills or action sequences or whatever. The reason being that, at that stage at least, they only had two gun props for the entire show.)