> Say they run into financial issues and cease operations.

Then the last thing they'll do will be to deorbit the satellites.

You also missed a major factor in your calculations -- the odds that a collision will raise the odds of further collisions. And that's zero. The vast majority of collisions are with centimeter size objects, which aren't going to shatter a Starlink. And if you collide with something large enough to shatter a Starlink, then it will remove enough energy from the orbits of most of the objects that they'll deorbit within a single orbit.