Okay I have to ask what was the original ending?
It was Spock who had to prevent Keeler from being rescued and allow the accident which killed her to occur, not Kirk. Kirk, hopelessly in love with her, could not bring himself to act and to let her die.
It's a little more complex than that. The timeline is altered when a Beckwith, a crewman who was selling drugs aboard the Enterprise, flees into the time vortex of the Guardians of Forever. Once in Earth's past, it is he, the vile, villainous person, who saves Edith Keeler from the truck accident.
When Kirk and Spock are present, Beckwith still tries to save Keeler.
Kirk is frozen, unable to act, paralyzed by his love of Keeler and the inner conflict within him over what to do. Spock is the one who stops Beckwith, allowing Edith Keeler to be run over by the truck.
Kirk, the hero, in an emotional lapse is unable to do anything. Whereas it is the "scumbag" Beckwith who, in a moment of heroism, saves Keeler. In the end it is Spock, the rational one, who does what's best for the universe.
Not everyone is what he or she seems. Not Kirk. Not Beckwith. Through the action of the characters, we see that human beings aren't always predictable from moment to moment. That even the best of us can succumb to not being able to make the right choice.
IMAO, that says more about human beings than any of the speeches on how "humanity has evolved"... blah.. blah.
Moreover, it ends with a quiet scene between two men who have shared much through this adventure. "No woman was offered the universe for love," Spock says to console his captain.
Had Roddenberry let that ending stand, imagine how much more of a biting, edgy drama Star Trek could've been. I don't mean in terms of the grit, dark drama of nuBSG, but stories which say something interesting about the psychology of our heroes, especially James T. Kirk.
Well, I gave the abridged version.

Not to turn this into a discussion of "City", but in defense of how it was finally done, if Kirk had acted as intended in that draft, I think he would've had to tender his resignation to Starfleet upon return to his time. Talk about being emotionally compromised at the critical moment. He could never be trusted in command, again.
As it is, Kirk finally finding true love and the agony having to give it up as he did made his act meaningful. The needs of the many, and all that. Keeler had to die, and he had to sacrifice real happiness.