SpaceX wants their Starlink constellation to crack 100,000, and there are plenty of competitors lining up who will surely want to have "the biggest and the best" at some point. None of those sats stay up terribly long, so you'll have a constant cadence of launches and reentries, both of which are a risk for their impact on the climate system, atmosphere, etc.

A largely unstudied risk, and once which will almost certainly be strenuously ignored for as long as possible.