The post was not directed at AWS. A "normal accident" (one that can't be foreseen, usually due to confluence of factors and bad luck) happened and they are dealing with it.

My post was directed at folks who said "web sites that went down as a result of such unprecedented EC2 problem made an engineering mistake by not building to be able to withstand."

Again - I am emphasizing "engineering mistake", not business mistake or funding mistake or resource allocation mistake.

My point is there are things you rationally protect against. But at some point, putting up defenses against more and more bad things stops being rational.

For different systems this point (where it stops being rational) is different.