You don't have to slaughter them, if you get regular heirloom chickens. The ones bred for corporate mass farms, yeah they are better off dead because they are genetic freaks either bred to only be meat and turn into a solid mass of meat that can't really walk or to shoot eggs out like a gatling gun, both of which are horrible for any potential long-term health. But traditional chicken breeds you can just accept having less eggs as they get older. Really the super genetic bred chickens aren't good for backyard growers to start with, you aren't going to consume that many eggs or want to have them so fat and meaty they can barely walk after 3 months.

My parents have had chickens for the last 40 years, never once slaughtered one. They either live to old age, or a predator comes along and grabs a slower older one or two.