Yeah. The movie had to end with him in the captain's chair. Period. The plausibility is problematic, and the practicality and reality can be (and has been) argued. But while it may have been a bit contrived, it wasn't so ridiculous under the circumstances. He got a heroic reward for a heroic deed. I'm sure it's hardly business as ususal for Starfleet to give the captaincy of a starship to a twenty-something, but besides doing what he did, Kirk is exceptional in almost every category. Spock Prime certainly seemed satisfied with what happened. Being in the captain's chair is his first, best destiny. He's a natural; an outlier. That's actually why I'm a little disappointed by the early part of STID.