The ending of John Wick (2014) closes the revenge story and leaves one door open: John kills Iosef, defeats Viggo at the harbor, and walks away wounded to adopt a new dog. He treats his own injuries and goes to an animal shelter, which shows him retired again, though the underworld still has a place for him.
In my opinion, the dog is the whole point. The film opens with a puppy that Helen arranged so John would not be alone, and it closes with John choosing a dog for himself. It could be argued that this is the only part of his old life he wants back, and that everything in between is the cost of getting there. The break in at his house, where Iosef and his friends beat him, kill the puppy and drive off in his classic car, is the one scene the harbor fight answers.
The ending also lays the groundwork for the series. Viggo's bounty, the assassins who answer it, and the Continental with Winston at the desk are all in place before the credits. Marcus is the quiet casualty of the plot: he is the friend Viggo hires to kill John, he hesitates and fails, and Viggo takes it out on him. In my view, that is the film's sharpest piece of writing, because it shows what a hitman's loyalty is worth before John ever picks up a gun.
Compare how the sequels end and the difference is clear. Chapter 3 ends with John excommunicado after killing a member of the High Table inside the Continental, and Chapter 4 opens with him recovering in hiding with the Bowery King after being shot by Winston. Those are endings about institutions and rules. The 2014 ending is about one man, one house and one animal, and, perhaps, that is why it still works as a self contained film. The Continental itself is only a place of rest here, and Chapter 4 later sees the Marquis destroy the New York hotel, which makes Winston's desk in the first film look very different in hindsight.
The stunt background matters too. A report citing Deadline in September 2013 said stunt coordinators David Leitch and Chad Stahelski would direct, making their directorial debut, and the harbor showdown reads like a stunt team's calling card: a clear geography, a practical fight, and a hero who ends it bleeding rather than untouched. Chapter 2, john wick 2, picks up the threads the ending leaves loose.