I feel like testing in prod builds the needed urgency to get things working faster. What would've taken a more winded path through regular unit tests, gets to production quality faster through the more important error cases that surface on prod.

Too many times, the issues we predict are going to happen, are not the most common ones or the ones that we really should have foreseen (hah kinda like the process to find product market fit). The best way to know what's wrong is to simply put it out there faster. And then persistent error-correction to make it better (there's never a best).