"...35 million objects this size (6 mm)..."
There's about 180 million square km of Near Earth Orbit area, let alone the dimension of variable altitude. So that's at least dozens of cubic km of NEO space per fragment. Even at 8 km/s, each is carving out a volume of 0.0007 cubic km per day, assuming a roughly 1 sq m death zone.
It's a bit like those aircraft tracking apps that give the impression that the skies are completely filled with commercial air traffic, but when I go outside and look up there are only one or two. It's because the app shows each aircraft not to scale, as if it were many km wide. Shown to scale, they're nearly invisible, and widely scattered.
If all of these 35 million objects were painted bright pink and gently dropped straight down to rest upon the Earth, and you went looking for them, it's unlikely you'd ever find one. Some people would find a few, but not you.
It's funny how '35 million' can be such a small number.
We humans still need to figure out how to clean up this mess in NEO.