How long would it take for orbit decay to bring these objects back into the atmosphere?
- below 350 km, months
- 400km a year
- 500km five years
- 600km ten years
- 700km twenty five years
Etc.
However, when a satellite breaks up, the small pieces are much more affected by drag. Another is effect is that an explosion results in approximately 50% of the pieces having a lower orbit. So even though the Cosmos-Irdium collision happened at 700km, which would take about twenty five years to deorbit, in just five years, more than a quarter of the debris had deorbited.
In LEO you have a completely different story. The distances are still large, but not as overwhelming as they are in GEO. Also, satellites are flying in every direction at very high relative velocities to one another. A piece of space trash could have thousands of potential collisions per day, so even if the odds of any one collision are small the cumulative odds start to add up over time.
Of course the other problem is that when we put people into space we put them in LEO. People have not ventured beyond LEO since the Apollo program. In the event of a full blown Kessler syndrome manned space flight would have to be put on hold for decades while we wait for the skies to clear up.
Particularly, since satellites in general are getting much lighter than they used to be (faster decay times, less mass in case of a collision, smaller target), as well as targeting lower orbits than we used to (most go to 400km-500km now).
Also, even in the event of everything in LEO blowing up, we can still launch to farther orbits / other planets.
It'd be risky though, since you have to cross LEO to get there (unless you're willing to pay the extra cost of launching to absurd inclinations and correcting in space).
It's not a game of Asteroids, but it's also not as thin as the asteroid belt.