If you blow up all the Starlink satellites, the little pieces start reentering the atmosphere most preferentially in very short order.

Kessler syndrome is more complicated than just "satellite debris bad". At the altitudes the Starlink satellites are at, it is a near non-concern. There is a causal link there... we can afford to put up lots of little satellites at that altitude for that exact reason, and that's exactly why they are where they are. That it's also less energetically expensive to launch is just a bonus.