This comment (I think, from my limited experience) really does capture all the context around my question really well. Of course not all old code is immediately

This comment (I think, from my limited experience) really does capture all the context around my question really well. Of course not all old code is immediately well-written by mere virtue of its survival - a lot of it is, as mentioned here and else where, sustained because of internal company politics or financial reasons, in flux and a living codebase, etc. But I am interested in the stuff that survives because of how well-written it is. To your point though, the context and ecosystem of these codebases is as important, if not more so, that the code itself. A lot of comments are pointing to the linux kernel which is a tour de force in and of itself, but also really interesting for the community/communities its inspired.