But there's so much margin for error on these specific parts. Even if the test only ensures 60% strength, and weakens the part by 50%, that's still strong enough.
A part being mildly out of spec might not be a vendor problem. A part being 20% strength is absolutely a vendor problem.
Why was this designed such that the failure of A SINGLE STRUT would be catastrophic?
I don't like to blame a vendor for what should be an engineering problem, whether this means design or testing.
The problem here --assuming it is as described-- is that someone designed a system with the assumption that none of these struts would fail. And, furthermore, executing on a design where the failure of ONE strut could cause a disaster.
Anyhow, that's what it looks like to me given what's been released.
Why was this designed such that the failure of A SINGLE XXX would be catastrophic?
There are plenty of things in a rocket that can fail that would take the entire vehicle with it. Structural mechanics is pretty well understood and loads are well predictable, so it seems perfectly reasonable to me to design with the assumption that a part with a 10x safety factor will not fail.
If you didn't, your rocket would never get to orbit anyway.
A part being mildly out of spec might not be a vendor problem. A part being 20% strength is absolutely a vendor problem.