Starlink is redeploying to 300 miles. Many consider Kessler to be impossible at 300 miles. Any unpowered satellite at a 300 mile orbit will deorbit within a couple of months. But a collision means fragments which deorbit faster because they have a higher surface/weight ratio, and because orbit disturbances lower that time considerably. Any single disturbance that raises aphelion lowers perihelion.

A smaller player like North Korea and Iran would not have as much to lose. Iran is doing something similar today, suicide bombing everything it can.

These LEO satellites are low enough that I imagine a Kessler situation would self-resolve within a few years.