[Still Thinking]

How are all the airframes not fatiguing and falling apart? Not that many cycles per year?

[Rockingham]

Limitations on maneuvers and g-forces reduce stress and help extend service life. In addition, airframes can be rebuilt. The B-52 is kept flying because it has an immense and flexible payload capacity and unusually low operating costs.

[YogicCowboy]

It seems to me that much of them are effectively rebuilt bit by bit throughout the timeframe - just not all at one.

For interest, see the movie:

No Highway in the Sky (1951), with James Stewart.

It is about a scientist who predicts aircraft failure due to metal fatigue.