So eating a healthy, balanced diet is being selfish?
I hate this argument on killing an animal. The animals are bred to be eaten. That's their purpose in life. If we don't eat chickens, then what purpose would they have? they are crap at flying, so all they would be is easy targets for predator animals. So you want these poor, defenceless birds to suffer the horrible experience of being chewed to death? That's terrible. You should save the chickens from such torture and eat them after they have been given a quick, clean death.
The scenario you described would indeed be terrible, but not very realistic since people just aren't just going to stop eating or using animal products overnight to then make all these animal products unsellable and then making the release of these animals into the wild into an option of any kind.
Instead we decrease the demand of these products so the number of bred animals also decreases, but for the benefit of basically every other being on this planet when considering how much environmental harm animal agriculture is causing with the huge amount of greenhouse gasses, rainforest destruction, topsoil erosion, ocean dead zones, and every other environmental ill its the leading cause of.
Harm to the environment we humans live in, while also paying for the process with either the large amount of subsidies given to this highly unsustainable industry, paying with our health with food-borne illnesses, rise of antibiotic resistance bacteria, and possibly increasing the risk of cancers, and paying for the environmental harm mentioned previously that the industry doesn't have to take responsibility for either. People losing their lives or livelihoods because of these environmental ills and climate change that they are a huge part in, and doing all this while also treating ton of the people who are working for many of these companies very poorly in the different stages of the process to also cut costs somehow.
Obviously not forgetting all the other animals that keep having their habitats destroyed or polluted and as a result becoming endangered or extinct.
To do all this to so many others in order to unsustainably breed these billions of animals with emotional and intellectual capabilities to simply have shortened, often miserable lives in subpar conditions and then simply be slaughtered and made into a meal. Doing this while living in civilization full of nutritious and enjoyable plant-based alternatives to eat on a healthy, balanced diet.