There's no one who would every consider decommissioning the dam and returning it. It has happened in a few small cases in the U.S., but it's pretty unique. By and large, once you have the dam the value of being able to store water does not go away—especially as when you increase the number of wind and other non-dispatchable sources, such as solar, you need somewhere to store that excess energy from time to time. You can't beat hydroelectric. So in the case of Mactaquac or any other hydro, occasionally you have to replace the rotating equipment, upgrade the turbines, and replace the generators within the plant itself, but the dam lasts a very long time and we should never even consider decommissioning the dam.
On May 21st, 2013. See this statement in context.