It would be great if someone like Dex was a multiple title winner but he will probably(and should be) one and done. Dexter is no spring chicken- late thirties?
Nothing wrong with reaching the pinnacle and then retiring! 
Coleman won at least two Mr. Olympia in his 40s and was in his early-mid 30s, when he won his first title. Chris Dickerson was in his 40s, when he peaked in his career, placing second at the Olympia twice '80 and '81 and finally winning it all in '82. His mentor, Bill Pearl, was at his all-time best at age 41, when he knocked off 3-time Mr. Olympia, Sergio Oliva (and a host of other stars of that era) to win his 4th NABBA Mr. Universe title.
So, Jackson being in his late 30s is hardly a sign that he's peaked. Even if this is his only Olympia win, Dexter has been to the mountain top. For at least one year, he's the best in the world. And, nobody can take that from him.