alt Hacker NewsStarlinks are in low enough orbit to passively decay in less than 5 years, that really can't meaningfully contribute to a Kessler syndrome.
Chinese mega constellations on higher orbits & their spent stages left in space are a bigger issues.
Still in case it got going & made higher orbits unusable, starlink would likely still work just fine on the lower self-cleaning orbits, not to mention using a partial (and hopefully soon full) RLV for replenishment.
5 years is still 5 years and Musk needs A LOT of payload for SpaceX to justify 1 Trillion dollars.
This 1 Trillion Dollar has to be translated to either sending up A LOT of foreign payload OR his payload; All of this payload = new Satelites. Its not like we are sending earth resource up in space to build a dyson sphere.