the system shouldn't be designed that way in the first place

Designed what way? Having parts from several manufacturers? Everything is designed that way. No manufacturer is an island, and having every manufacturer reinvent their own wheels is a terrible idea.

Tesla isn't going to write their own firmware for every component that they buy from another company and no one sane would expect them to.

when the vendor fucks it up due to releasing the product in a half-baked state

There are so many assumptions about what's going on in this statement that it's hard to even begin addressing them. It is not possible to test any device that will be used in the real world in every possible set of circumstances that it might encounter. This doesn't mean it's "half-baked", and it's not an "excuse", it's just the nature of reality. Best you can do is test the most common circumstances.