I dont get what the problem is with the ship being decommissioned so early, even if she was a new ship at the time it was christened as the A.
After the Khitomer battle Starfleet may have decided on the spot to retire the ship for a couple of reasons;
- The Consitution Class, and the Enterprise, represented Starfleet during the time of numerous conflicts with the Klingons, a conflict both sides were trying to put behind them.
- Its participation in the making sure the treaty was signed meant that they wanted to recognize this event and so decommissioned her instead of patch her up and send her off to get squatted by some random phenomena.
To me the events of TUC made the top brass mothball the ship as both a political move as well as a PR stunt. Take her off of active duty and replace her with a new top of the line Excelsior which was nearing completion, a new generation of ship to start the new era of peace and exploration which not only honours the Enterprise and her crew but also the Excelsior too.
Sorry to Shatner and any fans of Ashes of Eden (which I did enjoy) but I dont think Starfleet would plan on using the Enterprise for War Games or giving her to a foreign power but could see it being docked somewhere as a museum ship kind of in the same vein as the 'Mighty Mo'.
Other Constitutions I can see serving into the 24th century on milk run missions or supporting Starbases in obscure areas where a more modern ship would have been a waste of resources. This would be why we see parts of a Constitution Class ship at Wolf 359.