When you look back, you're looking back from the particular world where X became a requirement. If instead, Y had become a requirement, you would be looking bac

When you look back, you're looking back from the particular world where X became a requirement. If instead, Y had become a requirement, you would be looking back wishing you had structured your code in some completely different way. YAGNI then applies to the extent that you can't predict which of X and Y it will be in advance.