My guess on that was that the scale changed as a result of USS Excelsior. Maybe not in Kirk's lifetime, or maybe it was, but Excelsior was suppose to be the next big leap forward for technology. While I is never confirmed or denied if the transwarp drive worked as it was suppose to, or if they went back and installed a new kind of warp drive in the Great Experiment. Only that it eventually worked as a starship and gained enough respect that the class is still being used as flagships all the way to the Dominion War about 90 years after the lead ship was launched.
The other possible candidate would be the Ambassador-class starships ushering in the new design styling that would show up in the Galaxy-class and many other starships in the mid-24th century.
The other possible candidate would be the Ambassador-class starships ushering in the new design styling that would show up in the Galaxy-class and many other starships in the mid-24th century.