Sure, if a collision happens (unlikely while under control) then another collision happens (also unlikely, space is big) then sure some debris could go into a non-decaying orbit. That’s true for all satellites. Should we just not launch any because it could make things harder for other satellites?

Starlink is very unlikely to cause debris, and any debris it may cause, if any happens at all, is unlikely to cause any future problems because odds are it’d decay even faster. In the unlikely event everything goes wrong, it could cause minor issues, the same as any satellite.