This is basically impossible to determine based on how safe the car "feels".
e.g. if it has a 99.9% chance of doing your daily commute without crashing and without you intervening, you can monitor it closely from the driver's seat for 6 months and there'll be ~90% chance that everything looks fine and you never need to intervene. But then if you start napping in the backseat on your commute, there's a 70% chance you'll crash within 5 years.
There are enough people who FA'ed and FO'ed in the form of ending up dead, or killing others, that we're not stuck in abstract calculations or speculation.
