I think Antonia was just random, and random never works in this context. Like introducing a character, like a red shirt, or Hawk or Valeris, just to have them get killed or be the bad guy.
Having a random woman in Kirk's paradise is a big wtf, when there are so many women in his past that fans, and casual viewers might remember. Edith being #1.
So here, your girlfriend turns and says "who's Antonia" and no one has a clue, she is no one, "never heard of her". As opposed to "who's Edith", and you say "she is from a classic episode, Joan Collins".
No big deal either way but one resonates, the other is nothing, never heard of her, and I don't care if he stays or leaves.
Very much so. When we met the other women that Kirk's been involved with we actually spend a little time with them. We learn something about them even if it's just trivial. We see that there's something between her and Kirk. A shared experience, a chemistry between them, something. When we see Antonia it might as well be Walter Koenig in a dress for all we find out. Oh, she rides horses. Yeah, that's what's Kirk been looking for in a woman all these years. If we don't know anything about Antonia, why should we care if Kirk leaves her to go back to Starfleet? She's just random woman #472 in the big black book of JTK. We know more about Kirk's relationship with Deela from Wink of an eye. At least we know that they had sex.
A lot of it can be blamed on the Nexus. Guinan told Picard that it was "like being inside joy." and that they'd never want to leave. Picard saw a Christmas ornament and snapped out of it. Kirk wasn't worried about jumping a ditch on a horse and realizes that he's in a fantasy. None of what we saw matches up with what Guinan said. It was like being told a carnival ride was the scariest thing ever. You jump the first time something unexpected happens but you realize that it's fake after that.
Either that or never for Starfleet officers is equal to about 15 minutes.