Everything in low earth orbit (LEO) decays and falls back to earth due to drag; as there is a very, very small amount of air there. The starlink satellites are all in LEO.

Space junk is mostly a problem higher up. In the higher orbits, old satellites move themselves to graveyard orbits; places where nobody really wants a satellite.

Having one superconstilation up there is fine, my concern is that once there are 2 or 3 of them, the communication between them to avoid each other will become a mess

Slammed!