In 3 years of chicken ownership, I've lost 3 hens.
white silkie 1 yr old, no idea. Just dead one morning. From breeder.
Clarisse, was my 1st hen, a BO, hatchery, 3.5 years old. Comb turned blue, she was real tired, and died. I'd have to say cardiac or production hen old age.
Olivia, Blue Orp, 6 months old. Although I always saw her eat, she never ate enough. Under all those fluffy feathers was just skin and bones.
I don't think chickens, ESP hatchery chickens, were ever bred to live long, just produce like mad. Clarisse laid 6 eggs a week for 3 years. Even the day before she died.
Cynthia, like you, I still miss them terribly.