In a sense you are right. That state where you have the complete program as a mental model goes away when you stop for that session. But if you pick it up the m

In a sense you are right. That state where you have the complete program as a mental model goes away when you stop for that session. But if you pick it up the morning after that mental model could be reconstructed in < 1hr and you can finish the feature and deliver it as value to a user. If you created good tests (unit/integration/regression) that captures the gist of what the code should do, two-year-older-you or another developer have a good chance of getting a similar mental model in order to change the code with confidence. So i consider "legacy code" as a scale where you on one end go "The reqs have changed, better rewrite it all!" to "The reqs has changed, but I can confidently add this functionality and keep some of the unchanged behavior!"