In-universe: I've been thinking Kelvin Kirk would have been better off being some kind of special ops soldier that Starfleet could throw at situations. He was great in taking down Nero, he succeeded in getting Khan, and he took down Krall. But he sucked at following Pike and Spock's orders, he mishandled Nibiru (according to Starfleet), and then a few years later he was already bored of being a captain and wanted to become an admiral. I think he would have made a good MACO.