Agreed. Kirk's assignment is clearly for the V'ger incident alone.
That's how it started, but there's no reason it has to stay that way. Kirk certainly wouldn't settle for that after going to such great lengths to get command back. He could argue that his success in the V'Ger mission proved he was still the best choice to command the ship. And Captain Decker was no longer on this plane of existence, so there was a vacancy anyway. And really -- Kirk just saved Earth. He could pretty much write his own ticket at that point.
His reduction in rank is just stupid, Decker's even more so, but there's no reason to suppose that Nogura and Starfleet Command intended it to be permanent. "OK, admiral, you can command the ship during this emergency, and after that why don't you just give up your responsibilities as an admiral and stay there for good." That just doesn't sound believable. Or it sounds more like a punishment.
I see it as the price Nogura demanded in exchange for agreeing to Kirk's request. "You want to go back to starship command that badly? Fine -- then it'll be permanent. You'll have to give up the extra rank and privileges of an admiral. You can't have it both ways." From Nogura's perspective, it was punishment in a sense -- the cost of defying Nogura's wishes. But it was a price Kirk was happy to pay, because from his perspective, being stuck behind a desk was the real punishment.
I've always interpreted the "Out there... thataway" line as Kirk taking the ship on a victory lap before returning to his duties as an admiral.
Which is certainly not how the filmmakers of TMP intended it, because they were trying to start a series of movies that they hoped to continue themselves. It's not like they knew going in that they'd be replaced by a totally different creative team. And of course TMP's story was based on the pilot script for Phase II, so it was always meant to be the beginning of a new mission under Kirk's command.
After all, Kirk's "Thataway" line was followed by the onscreen caption "The Human Adventure Is Just Beginning." The ending of the movie was meant to feel like a new beginning, for Kirk and for the franchise.