"you're bound to conclude that nothing any individual does can be a mistake, so long as they do it consistently." This is exactly how languages evolve over tim

"you're bound to conclude that nothing any individual does can be a mistake, so long as they do it consistently." This is exactly how languages evolve over time, actually. A group of individuals makes changes ("mistakes"), and is consistent in the usage of the new ("mistaken") forms over a given period of time. If you didn't accept that, you'd be forced to conclude that we're all just very bad speakers of our modern languages' predecessors.