Thanks for reading!

> Isn't much of this made redundant by being part of an existing system?

I haven't found that to be true in my work. If you're only making a minor change to an existing system, then you may not need a design doc, but a significant change to an existing system has as much, if not more, complexity and ambiguity than greenfield development.

> Also, this level of detail is a recipe for being outdated once the issues, compromises and compromises starts coming in

I think this is what people typically get wrong about design docs.

I don't think design docs are a good medium for being the perpetual, living description of the system. I think design docs should capture the design at the time of implementation. You should modify the design docs while you implement the work called for in the design document, but once you're done with that work, you freeze the document and preserve it for posterity only.

The design doc is about a specific change to the system. If you need a doc to describe the high-level architecture of the system as it evolves, that should be a different doc.