Not in SpaceX’s case. The satellites are in low enough orbit to deorbit themselves naturally if they fail, although they do so faster if using onboard thrusters at end of life. Any debris caused by failure is also likely to deorbit quickly. Not true for higher orbits, like 800km or above.

As far as being visible to sensitive telescopes, that is true, but also not surprising. Telescopes are designed to detect really faint things, so even though Starlink is now almost always darker than the visible limit once fully deployed in operational orbit, it can still show up in sensitive astronomical surveys.

Not really. Image how many cars there on earths surface. But the earth is far from covered in cars. The surface area of even just geostationary orbit is much bigger, and there are a lot fewer satellites.

I think there’s a big difference between the satellites making little dark spots in the sky and making faint bright spots. Maybe not. Either way I don’t think it’s a problem. If they can cancel out the turbulence of the atmosphere, they can cancel out whatever signal the satellites introduce. But that’s optical. I wonder if having all that metal up there would interfere with radio astronomy?

I would keep asking that question even though they probably know what they're doing with that issue.