Lets say aliens came and offered you a deal. You can either not exist or live a life of luxury with all delicious food and mates you want, then after 1000 years or so you'd peacefully pass away just like a natural death and you are informed that they'd eat your body. I'm pretty sure quite a few people would be open to this.The choice is even simpler with animals since from all evidence they lack the capacity for existential dread.
Now tell, me what would be the most moral choice assuming chickens are sentient? To indulge them to a pampered existence several times their natural lifespan...and yes, at the end disposing of their bodies in the most practical and beneficial way, its not like a chicken cares what happens to its mortal shell in whatever poultry afterlife may exist. Or leave chicken minds to the terror of being torn up by a fox in the wilderness or to never know the joys of existence in the paradise future ethical meat farms may offer?