If you use your compressor daily or own a shop that's one thing. Most of us have one in our garage that gets used fairly infrequently. If you're opening the drain and nothing comes out that would be an indication that you're not needing to drain it very often. Even when you drain it it doesn't get 100% of the water out, especially in the horizontal tanks. My vertical tank has kind of a bell shape at the bottom where the drain is and does a better job here.
Fatigue is a problem here as well, its undeniable. We don't even know if that tank was over pressured. Some info on this:
Pressure Vessel Fatigue
So how many fail a year? There's risks to everything but you can either be scared as hell of everything or assess the risk and act accordingly.
