There is already some trash flying around in space from decommissioned satellites. Reentering the atmosphere needs some energy to initiate the process and that was/is not always planned for. The combustion during reentry is producing CO2. This CO2 emission will scale up when the number of satellites is scaled up.

The alternative of collecting debris in space is discussed by some space agencies. I really don't think it is a good behavior from SpaceX to put more trash in space and let public money take care of the cleanup later.