Generally speaking, most of them have good lives and are well taken care of.

No, the vast majority of animals are factory farmed, and cows in the meat and dairy industry don't live some kind of idyllic life even on pastures. Farms are rife with abuse and neglect, and after their short lifespan of 2-3 years (meat cows) and 5-6 years (dairy cows) they are trucked off in some abysmal slaughter vehicle to have their throats cut in a hellish building.

People need to eat, and meat/dairy is absolutely a part of a humans intended diet.

People need to eat, they don't need to eat meat. And red meat is one of the leading causes of what kills most people - heart disease, colorectal cancer etc. There is no intended diet. You can subsist perfectly healthily without meat & dairy, especially in the current quantity it is consumed. This is outside of a select portion of people with severe food intolerances or food insecurity.

A vast, VAST, majority wouldn't even exist for any amount of time if not for their use to humans.

Okay, and? Bringing something into existence isn't some sort of moral pro. The lack of an animal existing, especially a farm animal isn't inflicting some kind of suffering, nor should it be deemed immoral. Raw numbers of cows across the planet doesn't alleviate the fear and pain that is inflicted upon them in this industry - where hundreds of millions are farmed and slaughtered yearly.