If there is one thing that that agents/LLMs have highlighted, it is how much credit those test writers do deserve. Teams that were already following a TDD-style approach seem to be able to realize value from agents most easily because of their tests.

The tests are what enable: building a brand new JS runtime that works, rewriting a complex piece of code in a different language (e.g. Golang instead of TypeScript) that is more performant for that task, or even migrating off of an old stack (.NET WebForms) to something newer.