I think that is the "business" side of the story. Their food to laying ratio goes down as they age. But if they become pets, then that is different. What does a cat or dog give someone for the 15 years they are with us? Love, companionship, company, no eggs involved. And my dog eat a LOT of very expensive food compared to chicken food. If you count on your eggs for yourself, then maybe you could refresh your flock with new chicks every two or three years if you have the space? I have thought of this myself and have not had to make that decision yet. But I am attached to them as are my boys, so we shall see.