You need to recognize that sharing animations or skeletons
is not the problem. It never is, never was.
The way modern WoW adapts skeletons is very streamlined; practically all Humanoids share the same Skeleton in terms of how the bones are organized. And as long as bone names are consistent, you can transfer animations. So you remember my example of Hanzo from Heroes of the Storm? If I renamed the bones to match Reforged or WoW, I can transfer all animations so that Hanzo can use a Grunt's animations, or use Deathwing's animations, or use D3 Monk's animations. Of course it will look fucked up in some cases, but as long as bones are named correctly the animations can be transferred. That's why most of my work as a modder is literally cleaning up transferred animations, not actually doing custom animations or making models. A majority of my time spent is making sure custom models or transferred models don't have clipping. It is a very tedious part of the process, and luckily most of the time I only have to work in one set of gear to represent a new NPC, not have the unit customizable for THOUSANDS of gear options.
You can even see I replaced Hanzo's legs with Archimonde. The knee positions are not the same, but I can modify the skeleton enough so that the run cycles can still work and not look completely gimped. It's not a 100% foolproof transfer, and luckily I only need to make sure Run and Stand works, since those are the main animations that Reforged uses. I don't have to worry about fixing the legs for all the other HOTS animations, like his Jump, his Mount animations, his active Dodge ability, etc. So yes, skeletons can be modified to fit new race types. It does not mean everything else carries over though; you would need to ensure all gear still fits the character, and all gear combinations work with the new animations. Like even changing Hanzo's bow, I need to make sure the new bow is size-appropriate and make sure it does not clip with his new Archimonde style Legs.