I'd go as far as to say it was a melancholy ending for Don. Just think of everything that happens up until that point. He's finally shed all his baggage. He's hit rock bottom, to the point of being catatonic. No home, no family, no friends. He's toxic to everyone in his life, and he knows it.
Then finally, he connects with another human being. He has a moment of realization. The camera focuses on Don's face, and you see him crack a smile. Has he finally found the happiness he's been desperately seeking out his whole life?
No. The smile belongs to the idea for the Coke ad. Another pitch, another boost to his ego, another fix of his addiction. It's all part of the same old cycle for Don.
In the end, it's always been a show about addictions. The drinking, the smoking, the sex - all addictions to help the characters fill the void in their lives. Don's primary addiction is the rush of that one great idea, and the accolades that come along with it. But those are fleeting. And so sadly, his life keeps going in the same destructive cycle. No happiness to be found.