Ambassador, fits in more with T3, honestly. It fits that generation of ships more than T4--and the Venture has taken up that position.

The Excelsior, according to Rick Sternbach, was to be replaced by the Sovereign anyway in ST: FC's development. So even a Lakota-Refit would eventually have been replaced. So it makes little sense it's in T5. But I guess they didn't want a T2/T4 cruiser...

Besides, one of the first ships that was screamed about being missing right at launch was the Excelsior, so they just fed the people what they wanted too.