If you think common sense can ensure safety then you are super human. I have a 10 million line program that outputs an extremely complicated dataset with hundreds of parameters. Occasionally one parameter disappears... at a rate of 1/1000 runs. This is a bug, but the bug bypassed everything. The system has 100 e2e tests of "defense" but because the bug happens 1 out of 1000 times, 100 integration tests did not have enough coverage to find the error... so the program made it to production...
Now with static type checking. That program wouldn't even compile. The compiler can prove that the program is wrong rather than you going through hundreds of unit tests to try to catch an error while proving nothing. Proving a program is wrong is better than writing billions of unit tests that cover the entire domain of a function.
Your strategy of using common sense to go through 10 million (exaggeration of course) lines of code is an effective strategy, but not an intelligent one.
Now with static type checking. That program wouldn't even compile. The compiler can prove that the program is wrong rather than you going through hundreds of unit tests to try to catch an error while proving nothing. Proving a program is wrong is better than writing billions of unit tests that cover the entire domain of a function.
Your strategy of using common sense to go through 10 million (exaggeration of course) lines of code is an effective strategy, but not an intelligent one.