5. Constitutions, a dated design in the process of being replaced, are retired instead of repaired if they are severely damaged. Remember that Spock intended that Valeris replace him: "This will be my final voyage on board this vessel as a member of her crew. Nature abhors a vacuum. I intend you to replace me." So it makes sense that the ship was intended to continue beyond the current mission, otherwise Spock's statement is a meaningless gesture. So what changed? Battle damage.
Maybe, but I'm not so sure about that. The Ent-A was in battle, but it wasn't as if she lost her nacelles or had huge chunks of saucer blown away a la Wolf 359. She was relatively intact and fully functional at the end of the movie. While it's true that Spock and the others clearly were unaware of Starfleet's plans to decommission the ship, I think it's more likely that the politicians simply said, "We're unveiling this new ship soon, so we need the old one out of here."
Of course that begs the question of whether the 1701-B was always slated to be the 1701-B or was renamed when the name Enterprise opened up.
As with the above theory, I think the plan was to always name the new ship Enterprise, thereby necessitating the early decommissioning of the older one.