I'm going to say Children of Men, not because it's a very imaginative scenario (no-one can have any babies anymore), but because the implications of that are so well ingrained into the universe. There's the typical in your face products such as empty & abandoned schools, but there's also a backlash against immigration & the rise of a police state and government-subsidised suicide pills. Basically everything in the world looks completely like it has a reason for being there.