I like to think the old girl had a few years left in her, and that yes, without the damage wrought by Khan she would have been eligible for minor refit (bringing her internal systems up to at least be on a par with the Saratoga's internals in TVH) and then yes, send her out again, be it for another five year mission or even a role closer to home, if the SF Operations felt they needed to minimise wear and tear on a 40yr old hull.
ETA: The idea of an alternate timeline resulting from a diversion at this point in history is a fascinating one, it would have implications for the entire quadrant and beyond...
ETA: The idea of an alternate timeline resulting from a diversion at this point in history is a fascinating one, it would have implications for the entire quadrant and beyond...