And I would counter-argue that besides the saucer profile, the open-top nacelles and the round deflector, the ship is completely different than the Excelsior. But your hypothesis would certainly explain why there are no Ambassador variants. However, it wouldn't explain why the shipbuilders started with the Excelsior, upgraded to the Ambassador, but went back to the Excelsior in a major way later.
I put it to you, Mr. Okuda: Just why was the decision made to have Ambassador class ships with registries of 2XXXX, and Excelsior class ships with registries of 4XXXX? In early TNG, all the Excelsiors had low four-digit registries, but by DS9 they had these huge five-digit registries in the 40,000s!
Why? Why this madness???