Space junk is real but much less of an issue at the low altitudes at which these newer constellations usually operate (very low in low Earth orbit). Without expending propellant to boost orbits, atmospheric drag causes them to naturally deorbit in months or a few years at most. Space junk and orbital debris are much bigger issues for higher altitudes where atmospheric drag is much weaker, but none of these satellites are that high.
Yeah, the debris removes itself with time. But there is still some uncertainty around how deorbiting harms the atmosphere [1]. Immediate effects of a single deorbit seem to be quite low but having such deorbiting scenarios at a higher frequency could have some negative effects.
> Indeed, even in the worst-case scenario, the average annual global mean ozone loss is found to be between 0.17×10–4 % and 8×10–4 %, while the Antarctic local ozone concentration change can reach about 0.05%. Those impacts are negligible when compared to the impact of anthropogenic activities, in particular emissions of industrial halogen-containing substances such as CFCs, that caused a global ozone loss of 3-4% (since 1970).
