I'm not sure that ships are absolutely built normally just to be more powerfull than the ships before it. After the Iowa class battleship was built there hasn't been anything built with that kind of armor and gun size since. The Excelcior class to me was the Iowa class of its time. The hull was built up to support the experimental transwarp and the Federation had far fewer allies than enemies. As the federation expanded and still had the Excelsiors in service, is it possible that the new ships built were more geared to specific mission types? Have there been any difinitive "official like" stats on the "post excelsior" class ship designs?