Don't think in terms of wide-body vs narrow-body. Think in terms of long-haul vs short-haul.
The lifespan of a plane tends to have more to do with the number of pressurization cycles it goes through (and the number of take-offs and landings) - they go together, after all. Pressurizing the plane stresses the plane, and eventually the frame just can't be trusted, no matter how good the airline is at maintaining the plane. Short-haul flights (say going back and forth between New York and Washington D.C.) could conceivably manage 5 round trips a day or more, which is much more stress than a plane flying one round trip per day between New York and London.
Even most planes used for short-haul flights would be expected to last more than 10 years, and planes used for long-haul flights can be used for 30 years by major airlines. But keep in mind that airlines will certainly replace planes earlier if a newer plane has enough efficiency gains that will allow costs to recouped more quickly.
Singapore Air returning this plane already (and there's a second as well) means that the plane simply isn't profitable (or profitable enough) for them to keep flying. Most likely whatever route this plane was used for will continue with a smaller flight that costs less to fly.