When chickens get respiratory illnesses, they generally do not get well. Respiratory illness are usually something that renders all exposed birds carriers of the disease for life. Even if they do not have signs/symptoms they are busy infecting the whole flock to their illness. It is often recommended that you cull flocks exposed to CRD and do a thorough cleaning/sanitizing before getting any new poultry.
When chicken have reproductive issues, those issues are often fatal as well, but not always. These don't spread to the flock, though, so you can let them ride until the time comes when a bird's quality of life has deteriorated to the point where culling is a kindness.
Chickens are masters of healing traumatic injuries, though. They can withstand injuries that would render most of us dead at the outset. They can get torn half apart, fully heal, and still lay an egg everyday.
It is all about what they have wrong with them in the first place. Sorry for your losses.