Comment by karmakaze

There's another aspect of unit testing. It makes the units testable. The greatest benefit of this is that the units tend to be more coherent. A large blob of code that isn't unit tested may not have clear boundaries or a functional raison d'etre. Tests also serve as documentation or demos of the units which is great for onboarding devs later on.

Maybe AI analysis/synthesis will change the math on this, but beyond early prototypes and PoC's, tests pay for themselves.