That still leaves those qcow2 images that use features not supported by
qed. Just a few features missing in qed are internal snapshots, qcow2 on
block devices, compression, encryption. So qed can't be a complete
replacement for qcow2 (and that was the whole point of doing qed). If
anything, it can exist besides qcow2.
qcow2 is a feature-driven format. It sacrifices some of the core
qualities of an image format in exchange for advanced features. I
like to use qcow2 myself for desktop virtualization.
qed applies the 80/20 rule to disk image formats. Let's perfect the
basics for most users at a fraction of the {development,performance}
cost.
Then, with a clean base that takes on board the lessons of existing
formats it is much easier to innovate. Look at the image streaming,
defragmentation, and trim ideas that are playing out right now. I
think the reason we haven't seen them before is because the effort and
the baggage of doing them is too great. Sure, we maintain existing
formats but I don't see active development pushing virtualized storage
happening.