Seeing as the OP was not a question of morals but of meat being edible or not.... any type of slow cooking will make the meat "worthwhile". You can make a nice chicken salad out of old layers, chicken pot pie, chicken soup...ect. If that is the way you want to manage your flock, then go for it. It is the way the professionals do it. Chickens will lay their whole lives but after 2 years or so they are less productive. If you want to maximise your feed to egg ratio... eatting or at least getting rid of 2-3 year old hens is essential.