F.R.E.A. (p. 170, 9th ed.) describes Ext Obs. as " almost always considered incurable," and as "economic . . . when market conditions lower demand, as during a recession."
I've never addressed functional obsolence in the CA section of a 1004 in this manner and wonder whether others are doing so [based upon a hypoethical condition for the sake of argument that the CA is pertinent in the SOW.