If a satellite is in a very low orbit then any debris fall to earth quickly. Almost all cubesats have a orbital life of six to five years. The space station, in a normal low earth orbit, would hit the earth in around three years without reboosts.

The higher you go, the more absolute volume there is, the slower the satellites there travel and the fewer satellites there are.

So that leaves a band of mid-upper LEO satellites to consider for Kessler Syndrome.

When you have a collision between satellites, the velocity on average of each fragment is less than it was before, making the fragments decay faster. The fragments after each collision are smaller than they were before as well, thus having more air and solar resistance, which also makes them decay faster.

The Chinese FY-1C satellite that was blown up was the absolute worst case for a satellite explosion. It was located on the high side of LEO with a decay time measured in human generations, and it was in a super crowded sun-following orbit altitude, sharing space with lots of weather and earth observation satellites. The explosion generated 150,000 debris fragments.

In the ten years since, most of those fragments have reentered. We now have 2,000 golfball sized or bigger fragments being tracked, and over the past ten years, the only casualty has been a small, ten pound reflector satellite. Small debris pieces fall, and big pieces can be tracked and avoided.

If every satellite we have put into space magically exploded, and the debris was magically moved to a high LEO altitude and evenly distributed to avoid any holes, then we could still launch through this orbit of death to higher altitudes with only a 1 in 1000 probability of impact.

Space regulation has gotten much tougher since the bad old days. Satellites are now required to have deorbit or safeing plans, and the median satellites size has gone down by a couple orders of magnitude.

Kessler syndrome isn't something that keeps me up at night.