Unit Tests are not the one and only metrics for software quality. Believe it or not, before unit tests were coined, we managed to build reliable software too. Actually a lot of the internet was actually built that way.
But your assumption is very interesting, because it states a very common misconception of quality (whatever that means, reliability, maintainability, resilience, etc):
* "Unit tests imply reliable software". Well, it does not, you'd need reliable unit tests for that, and from what I've seen in the industry, not every unit test out in the wild increases reliability.
* "Without unit tests, you can't have quality software". That is just not true either. First, not all tests are unit tests, end-to-end (integration/functional/whatever) test are extremely useful and raises the quality too. Even without any automated test, the quality of documentation, contract programming, manual testing, etc also help a lot.
Also, note that apart from testing, a lot of different practices contribute to reliability. Separation of concerns, the language level of abstraction, the overall experience and involvements of the developers, and many others.
But your assumption is very interesting, because it states a very common misconception of quality (whatever that means, reliability, maintainability, resilience, etc):
* "Unit tests imply reliable software". Well, it does not, you'd need reliable unit tests for that, and from what I've seen in the industry, not every unit test out in the wild increases reliability.
* "Without unit tests, you can't have quality software". That is just not true either. First, not all tests are unit tests, end-to-end (integration/functional/whatever) test are extremely useful and raises the quality too. Even without any automated test, the quality of documentation, contract programming, manual testing, etc also help a lot.
Also, note that apart from testing, a lot of different practices contribute to reliability. Separation of concerns, the language level of abstraction, the overall experience and involvements of the developers, and many others.