> There's an argument to me made that the age/popularity of a belief system does correlate with validity, given that their individual memes are subject to Darwinism.

Absolutely not, because the fitness, in an evolutionary sense, of those memes does not correlate with their truthfulness but with how easy they are to believe because they appeal to our fears, hopes and dreams.

Take that "life after death" trope. The hard truth is that there isn't any, when you die, everything that made up "you" will be gone for good. But no religion based on that truth would ever thrive because the thought is just too scary and bleak.