Sorry, I wasn't arguing with you, I was adding on. Bicycle design is hardly at all like airplane design. I don't think most bike manufacturers do any real engineering at all - they just use what's available and what's standard with occasional destructive testing for quality control; and for aluminum that means they're generally over-built. The more boutique and custom makers who do design closer to the edge have to spend more time with engineers.

Now... knowing what you do about how fatigue happens and with a general understanding of loads on a bike frame; how would you evaluate the OP's claim that his previously undamaged seat tube failed from fatigue, not near a weld?