“when the final slight is extremely out of proportion compared to the previous ones, having that be "the final straw" becomes farcical.” I think we are arguing

“when the final slight is extremely out of proportion compared to the previous ones, having that be "the final straw" becomes farcical.” I think we are arguing past each other because this is exactly what I’m saying—the phrase is intended to be used for these exact farcical situations. You seem to want there to be a ton of nuance in an aphorism, which you will simply rarely get. The phrase is used to communicate a sense of someone not reacting and then reacting all at once—your arguments about the exact differential between different events in the sequence leading up to the breakage of the back are irrelevant because that is not the point of the phrase.