> Codebases are often evolved rather than designed, and just are the way they are without persistent reasons for things. A common reason would be "because it was this other way before."

Yes and no, I think. Codebases represent a collection of point-in-time decisions, and each one of those decisions had a reason. There's always a 'why' even if it's not captured.

> Imagine all the stuff that's probably in MS office, for which the reason is totally lost. There were reasons, but they're not really retrievable.

True - and that's Really Bad. That means you're stuck not able to improve or change certain things, because you don't know why they were done that way - and when you have a userbase the size of Office, you can't just say 'f it' and break things - at least not lightly, as a code-level decision.