First off, leather is not a simple by-product of the beef and dairy industries. Without the value of the animal's skin, the beef and dairy industries would not be financially viable.
But the cattle are certainly not leather's only victims. Much of the world's leather is processed in vast tanneries throughout India and other developing nations. Caustic and toxic chemicals like chromium used in the process are causing massive environmental and public health problems throughout the region, including massive pollution in the sacred Ganges River which is one of the world's great waterways.
Each year, it's easier to find stylish and high-quality leather goods that are both Earth-friendly and cruelty-free (to both animals and humans). Please consider replacing your leather shoes with these healthier versions.