I have worked on a farm where one half of the owning couple rehabilitated race horses. Upon arrival, they were very highly-strung and neurotic. None were ever shot, but they were quite hard work (because of their previous lifestyle) and unsuitable for pet pony homes. Thus, while I personally disagree with horse racing, any 'unsafe' animal would be shot for health and safety reasons.
I still work in farming. Our cows do not eat 'rubbish'. When they are calves, they have milk, straw and specially-formulated calf nuts which are designed to meet their nutritional need. When they're a bit older, they swap the milk for water and straw for silage (fermented grass). This is usually in the winter months, when the ground is too wet and muddy for them to be out (seriously, they shelter by the hedges and bawl to come in as they do not like wet, windy weather), and also because the ground becomes 'poached'. Silage is also given in periods of low rainfall, when the grass doesn't grow very well. We keep them outside on grass as much as possible. We keep mainly dairy cows, which are kept for as long as possible, and in any case, their meat would go for cheaper cuts (e.g. pet meat, mince, burgers, etc). The more expensive beef cuts are from beef cattle (e.g. Belgian Blues or Limousins), which have muscling and fat marbling that is better suited for the meat industry.
If farmers stopped farming and released their livestock, then they would become wild (and potentially dangerous) vermin. Cows are quite large animals and non-farming folk often find them terrifying.
Some countries eat horse meat. It used to be common in the UK, but probably fell out of fashion at some point, like rabbit meat. Rabbits were possibly considered to be 'cute and fluffy' and non-working horses are very expensive pets (or field ornaments...). Cows, sheep, chickens, etc. are not generally seen as being cute, fluffy or as good pets, and milk, wool, meat and eggs are good by-products.
Personally, I'd rather stick to a more natural diet of eating meat, fish, pulses, fruits and vegetables than UPFs. Also, milk is one of the most perfect things to consume, as it contains small amounts of all minerals and vitamins apart from iron and vitamin B. If I could have a viable smallholding, I would, but at the moment, it's not economically viable.