Shells so densely packed that escaping Earth will no longer be possible.
I have a different take.
NORAD today tracks about 20000 objects. Musk wants to increase that by almost 2000 more. I think musks lowest orbit shell is around 500 km. This mass space trash exercise will consume more than 10 percent of trackable space hazards by a single company, be a space navigation hazard for decades at least, and for what? Faster internet? Give me a break. That business model was busted by the globalstar and iridium follies 20 years ago. I honestly don’t see how musk got approval for this, except he paid off Obama folks and now legally it can’t be stopped.