Piper only works with CitC. Which is effectively a virtual file system when mounted locally. And when you have a monorepo that big, standard tools that want to scan an entire directory will stop working. Every part of the developer tool chain has to change to deal with code base that big.
I agree that Google should have bought github. The tech they built to scale Google Code I think would have helped a ton with GitHub.
They sell and support Google Drive on Mac and windows, and the web. Selling a VFS layer is entirely within their capabilities. Blaze/Bazel has already been exported. You're right that there's a whole stack, but if Facebook can make jj happen, Google could make hg happen.
I agree that Google should have bought github. The tech they built to scale Google Code I think would have helped a ton with GitHub.