My not-in-any-way-canonical take on the seemingly inappropriate high registration numbers is that a)Starfleet keeps a number of older vessels mothballed, b)SF refits them when a temporary need for more ships arises, and c) many of the numbers represent reactivated Excelsior hulls in which the original names have passed on to newer vessels.
I don't see why Starfleet wouldn't build new excelsiors though. The technology didn't change in the last 80 years. Sure the existing tech has improved, but there was no revolutionary new technology in play. Galaxy class is projected for 100 years...
If Lakota can be upgraded to be able to match a defiant class in firepower
, why not keep building them from ground up?
Until Sovereign class was built, which ship would fill the roll of excelsior better than excelsior?
Well, this is the problem (the bold one). Say Defiant is indeed a newer star ship compared to an excelsior class. Buuut..., it is smaller. And, logically, a smaller starship should be easier and cheaper to build. So why would they make a newer excelsior (that can't outmatch the Defiant) if they could make cheaper but powerful Defiant Class?
What Timo said.
One is a cruiser, another a frigate. Defiant is absolutelly useless for anything except war. This makes it severelly limited and dependent on other vessels. Its usefuleness is even hampered in a war because of it's warp 9.5 limit. It can't intercept effectivelly, it can't escape more powerful warships. Without a cloaking device, this becomes a serious handicap. It can't go too far from the station. Defiant is in my top 5 ships, but I would only use it during massed fleet engagements.
Cruisers are best ships in Starfleet because of their multi-role capabilities. How would a warship fare in "Best of the Both Worlds" where the crew needed a cybernetics lab?