You included nothing about how costs affect anything. You simply assert that you should always test prod on pristine production units.

There are plenty of times outside of software where production units cost millions or you can only produce them so quickly, or both, and where your extreme take would result in remarkable cost or a competitor eating your lunch.

Which is precisely why its not done, and in the real world tradeoffs are made between what really needs 100% assurance and not. Spending money or losing customers for 5 9s of reliability through testing when two are needed is not a best practice, and is often explicitly called out as such.

In the case of rivian, maintaining a significant fleet of expensive, pristine, exact customer spec (ie not debuggable) cars just to try to get 100% prod ota success assurance is unlikely to provide value vs getting 98% assurance and not doing that (by rough calculation, it stands at 98% after this incident).