Oldhen is right. You can keep em around longer than commercial facilities say, much like meat chickens, but they are inherently a meat bird. It's like buying a Great Dane (Danes hit senior at 5 years, normal dogs hit senior at 9). Giant animals have much shorter lifespans. I have kept cornish crosses around for 2 years through careful management, I imagine you could do the same for these... But they will not have a full, natural lifespan the way a normal sized bird will. All of my old CXs have died suddenly, I presume from heart failures, though they were very happy while alive. They also eat a LOT of food when full grown.
Maybe you can sell them to someone who is interested in the meat for a little more than you spent on them since you've been raising them out for a bit and use the money you get back to get some heritage breeds instead. A little research will go a long way into picking the breed that's right for you.