Lot of feed saleman around or academics who can't see reality past a good sales line, next they will be telling us we need to keep our chickens on concrete so they can't have access to natural foods and spoil the fine balance struck by commercial food.
To get maximum productivity from birds sure commercial feed will be easiest but that is not to say a more natural diet is harmful.
Even dogs can revert to living wild quite readily, and hogs, feral cats are a problem just about everywhere, heck I know a deer hunter who was asked to shoot a cow next time he saw it because the farmer had trouble capturing it and it wasn't starving in the wild. Where I grewup in NJ there was a wild goat for YEARS that was occasionally seen
Animals are a lot more resilient than people think, and in survival situations even people have enough instinct to start craving odd foods, organs they would not touch endup being the only part of a fish or something that they crave, seen the interviews with people it happened to.
Mother nature is fa better at managing the world than we and she still has ultimate control over what we manipulate.