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.
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.