Chickens are not real long lived except for a very few. And I really don't think you can do more for them, than give them a good life while you have them. Were they fed daily, did they have safe and comfortable surroundings, good water, and good husbandry? If the answer to that is yes, then you did enough, nothing to feel guilty for.
I know a lot of people treat their birds with all kinds of treatments, but I secretly think that most would either have gotten better on their own, or died anyway. There really is very little you can do with a fowl that greatly prolongs their life, it is more of the luck of the draw.
And it always seems to be your favorite. Some days it is like, why does anyone keep chickens... and then other days, why doesn't everyone?
Mrs K