This issue astronomers have with Starlink is not with increased light pollution per se, where the effect is negligible, but with the fact that when there are 10,000 of these in the sky at one time, it will be much more difficult to point a telescope and not have one of your exposures contaminated by a Starlink satellite passing through. Some telescopes will sit on a source for hours for the faintest objects and the odds of encountering one of these becomes worrisome.

Anything visible to the naked eye or even binoculars is honking bright to modern astronomical telescsopes, which are sensitive to sources millions of times fainter. Even if they're not directly in the field, off-axis reflections and glints can contaminate exposures. A train of these moving through the field would be a total disaster.

For those saying let's just build space-based telescopes, sure, but it's 10-100X more expensive, and even modest ground-based facilities produce 100GB/night these days, and you can't do that from space (yet).