I've kept broilers well past butcher age before. They usually live 2-3 years before passing away overnight one day out of nowhere. They tend to be surprisingly robust chickens during their lives.
Nothing inhumane about giving a chicken a longer, happier life under healthy conditions.
Most giant breed animals have greatly reduced lifespans. This is true as much of Great Danes and draft horses as it is of our giant meat chickens. It's a pretty normal phenomena.
And no, their skin won't randomly split open from rapid growth. Skin stretches to accommodate the body, no matter the rapid weight gain... Or else pregnancies would pop people open all the time. This is doubly true of prey animals who tend to have looser, baggier skin in the first place. If their skin splits open there is something else very wrong.
I do suggest considering a near-ground roostbar for the chicken. My CX have always likes to roost on the floor of the coop. They can develop strange problems with their breast muscle tissue.
Broilers tend to be sweet, calm and active flock members. I miss having mine around.