Most of the changes replace one model with an equally detailed one, or add a bunch of decals that are cheap in modern engines with clustered rendering.

Adding props is a bit more expensive, but they can be very low detail and still look good from a distance. Something small like a newspaper or plastic bag can be less than 30 polygons with a 64x texture.

Nobody's asking for ray tracing and complicated shaders. Just small tweaks that make a city look more natural.