The "romance" between Spock and McKenna was totally out of nowhere and very last minute. McKenna showed some signs of teasing Spock but to me that felt more like they were giving her lines that usually were reserved for McCoy (did McCoy during STC have any moments of his love/hate bromance with Spock?).Yes, her demise was contrived to answer the question why Spock leaves Starfleet after the 5-year mission, something not really addressed in “official” canon.
But Spock didn't show any interest in McKenna throughout all of the previous episodes, but we know he can feel human attraction when we see the episodes with Droxine and the Romulan Commander.
So this unexpected subplot felt very forced and unconvincing.