The final episode said that humans were building warp 7 ships, which given their new alliance with the Vulcans and all the rest who had to hand over their future tech to dim little backward humanity really makes you think that the Humans were just building Vulcan ships now. This "intermingling" totally and completely invalidates the opening credits... 2161 was the end of humanity standing on the merits of it's own history driving to the future. Now every human being going to school gets the benefit of a million years (Yes a million years) of Vulcan Astrohistory and other hard science.
Enterprise wasn't being decomissioned completely, it was joining the "mothball fleet".
The mothball fleet was one of two things.
A. A graveyard.
B. A fleet of ships with such awkardly slow engines that it's silly to send them out of Earth's solar system. Slow stody warp vessels are still great impulse powered gunships.