You want to talk about unrealistic command assignments in Trek movies? Let's see... how about dropping Admiral Kirk back down to captain and demoting Captain Decker to first officer for the V'Ger mission, rather than just having Admiral Kirk lead the mission with Decker's Enterprise
as his flagship? How about giving Kirk & crew their own replacement Enterprise
after they committed mutiny, sabotage, violation of restricted space, and the reckless destruction of Starfleet property? How about keeping the same command crew together in the same posts on the same ship (or its replacement) for over a quarter-century even after they've all reached captain's or commander's rank? We've already got plenty of precedent for Starfleet making command assignments in ways that by modern standards are quite capricious and illogical.