Sometime between the start and the end, there's often a mandatory burn it all down step required to more fully satisfy either (2) or (3). Watching hundreds of MB of data getting written to my HD for an apt-get update makes me 90% certain that Debian indeed needs to burn it all down.

To their credit, they started really early. And burning it all down seems to be an increasingly better option for purely extrinsic reasons; the late mover advantage keeps increasing, and better and better options keep emerging. A LMDB-based refactor perhaps? This is probably a pipe dream, alas: Debian developers must have a hundred different programs that all build and extend forward the original "make it work" technology, to make it survivable. And that's really my final counter to your cute linear notion of ongoing improvement: the mold is set, and there are dozens of things bound up in what has happened.