I have yet to see a cx that didn't look like it was ready to die when it reaches butcher weight. Definitely a better way to raise them, at least they can act like chickens for part of their lives. Once they get close to butcher weight though their energy level diminishes and you'll be lucky if they take the trip to their waterer.
Your supposed to restrict their feed regardless of whether or not you free range them, every hatchery recommends that.
There are allways exceptions to every rule. I've allways been the type to go against the grain. I believe the answer to raising these birds to have close to normal life spans is achievable. In Spite of what industry says. Do they have some genetic problems, of course but then so do I. I think it will all boil down to the strain used,diet and husbandry techniques used.I'm in it for the long haul. Time will tell if I'm on to something
Although I've never had one of these live birds on the property, I agree with you that they can have their lives extended dramatically by the means you set forth but I wouldn't expect one to live much past a year or a bit more. I guess that would be long enough to get some hatching eggs from them, having been covered by a breed with strong constitution. I've read about others getting that done but don't recall what the results were and I'm interested to know if anyone pulls it off.