Kessler syndrome isn't really that much of a risk specifically with Starlink (for now at least), as SpaceX seems to be doing things right despite Musk. They're in such low orbits that even with a catastrophic loss of control, they'll deorbit very quickly. The real risk comes as more companies and countries try to get a piece of the megaconstellation pie. Starlink in its own seems to be fairly safe and sustainable on its own, but that may quickly change when communication for collision avoidance maneuvers needs to be international.

Despite Musk's well-earned reputation for being a shithead, SpaceX has this far been doing the right thing far more often than most other space companies, and while it's certainly possible that will change, the Starlink constellation will entirely disappear very quickly without constant replenishment, so it's not as if we'd have no chance to act if they begin to show signs of concerning behavior. What's far more worrying to me in terms of Kessler syndrome is the recent escalation around space warfare, as tensions between Russia, China, and the US continue to boil and nobody seems willing to really commit to making space a neutral zone. Even with space historically being an area of strong international cooperation despite politics (just look at the ISS), that unfortunately seems to be rapidly changing.