With all due respect Fenway probably needs to be replaced more then the stadium does.
Yankee Stadium is not the same 1923 building. It was gutting to the bone in 1974. So it really isn't the house that Ruth built. The building is old and out of date. Bathrooms are far and few, and the city as been sued many times for the building not being handicap accessible.
But the main reason is luxury boxes.
Fenway isn't going anywhere. They've been making loads of improvements each off-season since Henry & co took over. This year they've replaced all of the bleacher seats, replaced what were the "temporary" boxes that were set up for the 1999 All Star Game, and they're building a restaurant under the center field bleachers. I've read some thing that they're going to replace all the old wooden seats in the grandstand by 2012 (the 100th anniversary).