Leather is a natural byproduct of the dairy and meat industries
Cattle are and will be never slaughtered for their hides.
They are bred for dairy and meat production, so as long as there is demand for these products, there will be cattle hides. These hides are typically transformed into leather, giving them a new life that often lasts for decades. Hides that are not converted into leather are used for gelatin or end up in landfills, emitting methane, a potent greenhouse gas. There are certainly various alternatives to leather that perform very well, but these often rely on fossil resources. In other words, replacing a leather upholstery, which by definition uses a byproduct as its raw material, with plastic means additional fossil resources need to be utilized.