Thats assuming the average person buys six cables per year; multiple cables is likely, but multiple being replaced each year is highly unlikely. You are correct that you need to factor in extra cables improving statistical reliability, but one cable per iPhone gives you a baseline of total number of cables in use.If the average person buys six cables a year the number of iPhones and iPads sold tells you nothing about the reliability of the cable.
Another consideration is what constitutes premature failure; how many years of regular use after purchase is acceptable failure.
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