Kessler syndrome actually isn't really a huge concern with Starlink, as those satellites (and any debris) are in such a low orbit that they will deorbit on their own (within months or a few years) if control is lost or the satellite breaks apart. There are specific orbital distances where Kessler syndrome is a concern, where the satellites are too far away to be deorbited without assistance but close enough that they could interfere with new launches (unlike a graveyard orbit, which will never deorbit but is far away enough to not be a major issue.)