For something that foundational, you have to move as one giant, lumbering beast. Recall that Apple had at least one (that we know of) false start. A fair bit of the OS (CoreServices; Carbon, and the apps that still rely on it) still relies on HFS+-specific data structures and features.

Core Storage is already moving forward as an effort to dissociate the bytes in logical storage from the bits on physical media. In the short term, we might be able to sandbox the evils of HFS+ by having it just be a software abstraction on top of a much more modern file-system-like-thing. Yosemite already moves any machine it upgrades to Core Storage, even if you don't use FileVault.