YAGNI is not an excuse to never design as some critics have characterized it. If you need it, build it. YAGNI is a meditation on timing. Building structure too soon is as risky as building structure too late.

First bill: optionality

1. When you build structure before the feature arrives, you're committing on a guess.
2. The feature you prepared for usually isn’t the feature that shows up.

Second bill: NPV

3. Money has time value. Structure you build now for a feature due in three months is cost pulled forward and revenue pushed back.
4. The bill comes due even when your guess is right.
5. The cost of typing the code is not even considered. What about maintenance?