Guys,
Personally, I think the Nebula class WAS designed as the long term replacement for the Excellsior class, just as the Intrepid was designed as the replacement for the Constellation class.
However, Post Dominion War, Star Fleet will take at least five to ten years to return to it's pre-war strengths and role, so for at least five years the Excellsior's will remain in service, at they are fully replaced by either the Nebula class or a new design.
"Those Klingons are up to something, I can feel it in my bones"
Cmmdr JT Wayland
Strategic Operations Officer, Starbase 415, Dragomer Sector, Klingon Border, 2372