I have raised Cornish Cross chicks for the last 8 years. The last 4 or 5 we have left 1 or 2 I butchered. The oldest one I butchered was 18 months old. We have had them laying eggs even.The rate of growth on a CC is INSANE! I had one get mixed in with my egg layer chicks one year, didn't have the heart to eat it, it could barely walk at 12 weeks and week 14 dropped dead.
I will say that we have had a couple drop dead, probably of a heart attack, but mostly they do fine. I have had 1 pair that the rooster, also a CC, broke the hens leg trying to mount her. ( because he was so heavy)

The difference I have found is that when we let them roam over the huge enclosure with the other chickens and we don’t give them food 24/7 so they forage more. They are also able to fly up to the roosts to sleep but not all of them have been able to manage this feat.
It is vastly different from how I raised them in the beginning. Much more humane this way.