Yeah, I said wp-admin theme. It's not generally considered a theme but that's what it is: the core-provided view of everything you can touch when you're an admin. Try thinking of the back end the same way you think of a front-end theme.
Unfortunately WordPress has not yet devoted a development cycle to making the entire admin area easily pluggable. Any seasoned WordPress developer could hack together a plugin that hijacks wp-admin URLs and displays its own interface. Several have. Wouldn't that satisfy your requirements?
Of course none of that addresses the issues around efficiency and performance. My hope is that WordPress will finally undergo a deep refactoring to remove its worst practices even if it means forcing updates to popular plugins and themes. There is no longer a reason to fear mass defection from the community for pulling that trigger. After the initial shock the changes would be embraced and everyone would enjoy better performance.