It's ~5 years for intact satellites to deorbit from Starlink's altitude. Debris will generally have a higher surface area to mass ratio due to the square-cube l

It's ~5 years for intact satellites to deorbit from Starlink's altitude. Debris will generally have a higher surface area to mass ratio due to the square-cube law, so should experience proportionally higher drag force and deorbit somewhat faster. But the real key is that debris doesn't have to deorbit all the way to stop kessler syndrome, it just has to get a few km down below the orbits of active satellites, which will be much less than 5 years. I believe it would be difficult to trigger Kessler syndrome even intentionally at that altitude.