> But it's even harder for code to answer the 'why not' question.

Great point. Well-placed documentation as to why an approach was not taken can be quite valuable.

For example, documenting that domain events are persisted in the same DB transaction as changes to corresponding entities and then picked up by a different workflow instead of being sent immediately after a commit.

I don't think this is enough to completely obsolete comments, but a good chunk of that information can be encoded in a VCS. It encodes all past approaches and also contains the reasoning and why not in annotation. You can also query this per line of your project.