If a given debris item strikes, on average, <1 other objects before deorbiting, then the Kessler cloud will (eventually) disperse, absent new objects being inserted.

If a given debris item strikes, on average, 1 other object, the Kessler cloud is self-sustaining (or at least until particle size drops to some minimum).

And if a given debris item strikes, on average >1 other objects, the cloud grows and the collision rate increases with time, again until the material is sufficiently dispersed and/or eventually deorbits.

Launching new satellites sustains this equation.

Accidental or intentional debris-field creation of course greatly exacerbates it.

(What the theshold is for >1 collisions I have no idea, I suspect there's research on this.)