A) In my experience, writing unit tests makes me go faster as a developer, not slower. Why? Because I’m not spending time debugging.
B) No-one ever said that unit testing was all you need to do. At the very least, you still need end-to-end tests, and you need manual exploratory testing for the qualitative judgement only a human can give you.
C) With manual tests, you still want to automate tests for regressions; manual regression testing becomes a massive time sink over the life of a long product with multiple releases. If you’ve already invested in a testing framework, testing regressions is cheap.
Can you buid software without unit tests? Oh, you bet. I’d just argue that it is less efficent. I’d also argue that I’ve seen lots of test environments that don’t “take like 100 man-hours of setup time”, don’t “suck down a ton of engineering resources”, and prevent “any particularly relevant bugs.”