The argument is basically the following:
(1) Consider the claim that getting to 100% reliability is important.
(2) If the claim were true, then it would be important to go from 99.999999999999999% availability to 100%.
(3) That is obviously not important, so the claim must be false.
(4) Since we have shown that the best target value is not 100%, it must be some number x, with x < 100%.