Could you show an example that exposes a significant overhead of the current implementation?

There were reasons of using os.scandir() in os.walk() and glob.glob(). You may scan thousands of files and performs a heavy I/O only with few of them. There was smaller reason of using it in shutil.rmtree() -- removing an entry from the directory is fast, but likely you will spent much more time for creating a tree.

But I don't know reasons of using it in shutil.copytree() in which you need to performs a heavy I/O with every file and directory.