There's already so many starlinks in some bits of LEO that a single sat breaking up will lead to a runaway chain reaction that breaks up everything in that orbit quicker than the debris will decay out of it:

> The results indicate the current population of intact objects exceeds the unstable threshold at all altitudes between 400 km and 1000 km and the runaway threshold at nearly all altitudes between 520 km and 1000 km.

https://conference.sdo.esoc.esa.int/proceedings/sdc9/paper/3...