I keep hens for practical reasons.  Keeping hens until they die of old age has several negatives, including...
1)  Cost of feed.  The good layers eat plenty of feed as is, so much that I've quit selling eggs at $2/dozen cause I ain't making any money.  I'm just gonna keep a small flock for eggs/meat for myself.  An old hen that's not laying (or only laying an egg or two a week) is not paying her way.
2)  Space.  I like to get a few new pullets every year to keep egg production up.  If I were to keep them all until they died of old age my coop would become way overcrowded and the area around the coop would be eaten down to the ground.  In short, the hens would be miserable.
3)  Getting rid of the old hens also helps break up the life cycle of different parasites.