Shouldn't most of the pieces from a collision in LEO crash into the earth? If you consider a plane tangent to the altitude of the object, most pieces ejected below that plane should reenter and burn up. Any piece ejected outward will be in an orbit that will return to the collision point from underneath and must therefore hit the earth. Only parts ejected nearly in-plane will have lasting orbits. That's not to say it wouldn't be a problem.
That's right, collisions in LEO are not a problem. All objects in LEO are being slowed by atmosphere, the smaller and less dense the object the faster.
If two objects collide in LEO (I really mean close to Earth at this point) the perigee of any resulting fragment must be at the same distance or lower so even if resulting apogee is high the object must spend at least some time close to Earth where it will be captured by atmosphere.
The real problem are objects that collide above LEO where it is possible for collision fragments to have orbit that will never hit appreciable atmosphere and they can orbit for hundreds or thousands of years.
In the context of Kessler syndrome, objects that are doomed to deorbit after a collision can still be a "real problem." They just need to remain in orbit long enough to hit something else to contribute to the domino effect behind Kessler syndrome.
It's effectively an epidemiological transmissibility or chain-reaction question.
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.)
