Still a lot.
* NEO's (asteroids) are really only detectable at twilight so this would severely hamper efforts here, and I hope most people would understand how important NEO detection is from a practical, human-survival standpoint.
* Depending on your latitude/day of the year, "twilight" can constitute the majority of the observable time...
* e.g. for LSST 30% of all images will have a StarLink trail. If Space-X can somehow manage to get their satellites down to 7th mag (15% of their current brightness, 50% of the brightness of their "DarkSat" version), then LSST can mitigate a lot of impact; it will come down to extra cost + observation time for many science cases, however a lot of science cases are still SOL even with these mitigations (precision cosmology, NEO detection, etc.).
The point I'm trying to make isn't that astronomy is "doomed" (though some subfields will be) it's that we should not be in a situation where we are scrambling to fix problems like this (i.e. Space X is now helping mitigate things voluntarily, thank god, and only after a huge outcry). There should be regulations that ensure everyone is on the same page and that prevent issues like this and ensure at least a bare minimum set of requirements are met.
Those regulations are not there only because when these laws were written, nobody even dreamed of a situation like StarLink or Kuiper. I don't think Space X or amazon are "evil" or anything, they're doing great work, but they have zero incentive (except maybe PR) not to trash billions of dollars in astronomy research, and that is neither good nor necessary.