On the contrary, I've spent months recently working on a system loaded with tech debt that has fallen victim to a half done rewrite. I've used a debugger on it

On the contrary, I've spent months recently working on a system loaded with tech debt that has fallen victim to a half done rewrite. I've used a debugger on it plenty of times. It was only when I added proper logging to the critical junctures in the code that I could finally see very clearly how it behaves and see what input results in what code paths and output. I learned more in an afternoon than I did in a month.