Don't cull. I've saved many animals after vets recommend euthanasia or amputation or otherwise forecast doom. They seemed so bad I was afraid they'd die, but I forced my mind to believe my prayers had worked, and then continued the nursing care believing their healing would manifest.
Except for a percentage of poops being watery, mine are all looking frisky and healthy now. Don't give up. There were a few whose beaks I had to keep dipping in water every hour or two tonget them to drink, but they're super happy now.
You could call a vet and ask if you can add an antibiotic to the Corid water just in case.