Before tractors, almost the entire food chain involved animal slavery, since farms used draft animals. For that matter, even transportation was based on animal slavery (horses). Many of the sustainable and high performance fabrics like wool, silk, and leather are now replaced by synthetic fossil-derived plastics. But petroleum is also an animal product.
To be vegan, is to choose fossilized animal products and services over fresh.
I always wonder how many people who think eating meat is murder, evil, etc. have actually watched animals die in the wild. If I had to choose between getting knocked out with a stunner or being chased down by a predator, struggling and screaming while being torn apart by teeth and claws until finally losing consciousness from blood loss and pain - which is how wild animals tend to die - I would absolutely pick the slaughterhouse any time. From that standpoint alone I would absolutely call eating meat the lesser evil.
Of course death is only one dimension - many domestic animals live in terrible conditions - but mistreatment isn’t necessary for vegans to condemn using animal products. Even eating honey is frowned on, simply because bees are animals. I don’t think that simple flat rule takes enough information into account.
The problem with factory farming isn’t the end of the animal’s life, it’s everything that comes before then. Factory animals live absolutely miserable existences.