> It was all tested, repeatedly, over and over, and over the course of about fifteen year
So, you do test how your system actually works, and not just isolated unit tests.
> Again, when you begin knowing that the software has to deliver on such a range of systems, where one customer is two college kids in a basement and one customer is valued in the tens of billions (although doing a lot more, of course, than just what our software let them do), design and good software engineering goes a very long way.
Indeed. And that good engineering would include a simple wisdom "unit tests are useless without integration and E2E tests, otherwise you wouldn't be able to run your software anywhere because units just wouldn't fit together".
And once you have proper integration tests, 99%+ of unit tests become redundant.