A route gate that earns itself

The After Story lock is the most load-bearing route gate in Key's catalogue. It refuses to let you see the second half of the book until you've finished Nagisa's route — and it's right to, because After Story reads as a continuation, not a reward.

The heroine routes before that are genuine side-stories with their own emotional logic. Kotomi's and Fuuko's especially are not training wheels for the main route. Play them for themselves, not as prerequisites.