The Cornish Rock meat hybrid is bred to be butchered at around 6 to 8 weeks. They are not bred to live longer. Basically that is their lifespan.
You can get them to live longer by restricting their feed. You need to find that feed level where they don't put on so much weight they die but can still be reasonably healthy. A lot of research goes into how to feed and manage the parents and grandparents of the Cornish Rock meat bird. The parents and grandparents are not the Cornish Rocks but they have to have those genetics, just not in the same combinations as the hybrids.
You don't have the advantage of that research. To keep then alive you have to guess how much to restrict their feed. You are guessing on how to keep them on the brink of starvation where they will always be hungry but not starving them to the point of sickness. Having them forage for most of their food will probably work out better for you than if you providing everything if you have good forage.
It's your decision. We all have our own values. You need to follow yours, not mine or anyone else's. Just yours. Same thing with goals. Follow yours, not mine or anyone else's.
If you are trying to breed them to a dual purpose rooster to improve the meat qualities of your flock, there are people in the meat bird section of this forum with experience doing that. You might start a thread over there asking for that specific help.
@wyoDreamer I wonder if your egg laying issues didn't have more to do with diet than heredity. You would think that a commercial flock who's job it is to lay hatching eggs would be bred to lay well. Not necessarily large eggs but plenty of hatching eggs. That's the parent and grandparent flocks. I'd think some of that research would be how to feed them to get the maximum number of eggs. We don't have the advantage of that research.