The ending to Mass Effect 3 was compleat and final. In order to understand this fact, one must look at the entire trilogy in the context of the stargazer scene. The whole Mass Effect story actually starts with the stargazer, he is telling the little boy the legend of "The Shepard." In other words, the entire Mass Effect story from Mass Effect 1 through Mass Effect 3 was a recounting by the stargazer of how Shepard chose to break the cycle of extinction and how he allowed the species of the galaxy to evolve on their own path. There is no doubt that harbinger/ the catalyst was in fact trying to indoctorinate Shepard during the end game, this was the crux of the entire mass effect experience. Here Shepard is presented with a choice, break the cycle or continue it (this is very reminiscent of "this has all happned before and it will all happen again" theme from Battlestar Galactica) By choosing the destroy ending Shepard breaks the cycle and truly frees the entire galaxy. This was an event worthy of a legend and that's what the stargazer is doing, he's recounting the story of a true legend. Also, at the end of the stargazer scene, the little boy says "Tell me another story of the Shepard" to which the old man says yes to. This statement forshadows a new adventure with Shepard, but it most likely will be a story of shepard's earler life, like his exploits in the first contact war. Also of note, the planet the Normandy crashed on is the same as the planet the stargazer is on, which means the stargazer and the young boy are descendants of the Normandy crew. This may seem like a let down, but it all fits with the greater Mass Effect mythos.