I think it's important to consider something you mentioned in response to an earlier post in this thread: the inherent behavior of each species. Sometimes people complain that their bird is screaming when in fact it is making a call that is made by that species in both captive and wild settings. The call may be used during certain times more than others, such as when looking for a mate for example.
To think we can eliminate natural behaviors because they are annoying to us is not realistic. Using behavior modification to help a bird adapt to our household makes perfect sense, but to attempt to eliminate an identifying trait of any species is unrealistic.
Young birds use noise to get their parent's attention when they are hungry. Once full, they don't make that noise. Females have a special sound they make when asking to be fed by their mate. Males often have an overpowering vocalizing they use if their flock is in danger. Unless we see these things take place and relate the noise to the situation, we might never know that a certain sound has a unique meaning, just as a bird's body language can tell us so much.
I know that it is much easier for me NOW, to recognize the subtle differences in my birds' vocalizations than I ever could have when I first began keeping birds. I look back and realize that the conures I owned in my 30s and threatened with becoming parrot stew were vocalizing loudly only twice a day and out of joy, rather than "screaming" like I considered it then.
We do the best we can, with the knowledge we have, and hopefully learn as we go along.