Leather is a by-product of dairy and beef production, there is vastly more leather than we use for garments. Most of it gets processed into pet food or makeup or automotive lubricants or who knows what
You are on the right track. Hides are a byproduct. Nobody kills animals for them.
Once the hides are turned into leather, they are no longer biodegradable.
Veg tanned leather is impervious to bacteria. Fungi can damage it, albeit slowly.
Chrome tanned leather is similar but way more resistant. Probably 99% of the leather (except shoe soles) people deal with is chrome tanned.
If you try to put it in your compost you are going to be sad.