When I was a kid, by dad bought a bunch along with some straight run laying breeds. When time came to butcher the Cornish, we begged him not to because they were our 'pets.' They ended up living, but they were obviously miserable. Hens never layed (and we had them for a few years), and I know this because they were so fat they couldn't jump more than their own height and definitely couldn't get into the nesting boxes...with no eggs ever on the ground.
The roosters got absolutely massive. I've never in my life seen a chicken that big. Bigger than our Pekin duck, and probably about as fat as a turkey, minus the long extremities.
They all could walk (surprisingly) but when they ran, they looked ridiculous. Their legs couldn't bend, so just imagine that. Haha. Another problem was that the hens couldn't run away fast enough, and the roosters (cornish and the other breeds we had) would mate with them so much that they had gaping holes in their necks (that healed). We always separated the hens and roosters, but the light roosters could jump over and would mate with the cornish because they couldn't run away.
I refused to eat my pets after he slaughtered them after a few years, but other than my dad burning them all on the grill, he said they were incredibly tough. (I'm older and wiser now, and wouldn't have a problem eating my chickens...which I plan to do when I cull my 10 roosters
).
Moral of the story, butcher them. They will be miserable if you don't.