my only surmise is, no new excelsior class ships were made since they're time, but they still remain in service, cause its cheaper to refit the ship I imagine then to rebuild from scratch, not to mention in games such as star trek online, the excelsior is classified as an advanced heavy cruiser, even though the sovereign class is an assault cruiser, they still hold their own in that game.
Excelsior, and her sisters, was a wonderful design: Elegant Sleek and Powerful.
Basically she, with a few refit Constitution elements, seems like an older sister (at the very least spiritually) to the Sovereign class, cause no other federation classes til the Sovereign following her had a sleek look to them.
It puzzles me why Starfleet keeps the aging Excelsiors around. By this point the Excelsior-class must be, what, 120 years old? Yet they still compose 15% of Starfleet vessels. Same thing with the Mirandas.
I like Ambassadors much better, and they're 50 years younger.