Old growth is not "useful." It simply is. It needs to be left alone to work out its own salvation, and, John Muir might say, our salvation as well. 

And the reason why the replanted tree farms are overcrowded and fire prone is exactly as you say: every mature tree cut was replaced with 10 seedlings--that's the definition of a fire prone landscape, choked with trees and brush. 

As I understand it, the Douglas Firs are not even the climax species on the west side of the Cascades; they're colonizers whose natural role is to provide shade for the less-light-loving species that made up the climax forest before intensive logging let the light in--western red cedar, Port Orford cedar, varieties of hemlock and balsam among others.