Trust-by-default is a consequence of the fact that enough people don't exploit norms, don't lie. We're in no need of turning off the gene (assuming it's nature, not nurture, which I'm not so sure about), since if society found itself in a lie-dominant state (as opposed to truth-dominant), we wouldn't be as trusting anymore. (It's self-correcting already, without any need for manual override.)

It'd also argue that it's a lot harder for the majority of people to convincingly act and lie than you think (especially when they don't feel desperate or have grown up in a safe environment that doesn't force them to learn how to behave contrary to their natural instincts).