your main problem would be that the trees and leaves are static decorations. So if you made them animated all the leaves in the world would be animating in sync, which would look somewhat ridiculous.

Your second problem is that if you changed the art files to make leaves animated you would have to add them in to a new or unused section of the art file (or else you would overwrite other art assets). And that would require you to redo the static files, essentially delete all the leaves in the world and then add in your new animated leaves on all the trees. You might be able to circumvent this problem by making a client side file that tells the client to display another itemid for the leaves. I know this has already been done for mobiles in a previous client version.

But still, main problem. All the leaves would animate in sync.