You don’t get to freely acknowledge counter-examples and then magically say they “prove the rule” as if they don’t contradict your argument. THAT’S NOT WHAT THAT MEANS. And it’s quite all right for me to respect the elderly in general while acknowledging the baby boomers didn’t heed environmental collapse, stole all they money from future generations, raised their cost of living/education while keeping their wages st…
Interestingly, Wikipedia has an in-depth article on the usage of this phrase: https://en.wikipedia.org/wiki/Exception_that_proves_the_rule. It's stated that this phrase is derived from a legal principle of republican Rome: exceptio probat regulam in casibus non exceptis ("the exception confirms the rule in cases not excepted").
So, in this sense, the fact that books with old protagonists are rarely bestsellers proves the "rule" that the author stated.