Starlink is at a low enough altitude that it's largely insulated from Kessler syndrome. Even if the satellites fail, they'll deorbit in 5-10 years; that doesn't sound like a short time, but space is very, very big; it generally takes a long time for collisions to happen by chance. That's fast enough that there is very little debris at that height.
The real danger comes from constellations like OneWeb or Telesat Lightspeed (planned) where the orbital altitude is so high that failed satellites (one has already happened for OneWeb) will remain in orbit functionally forever.
