You've given a lot of consideration to modern recently created code. But the best modern recent code goes on to become someone's legacy nightmare. (The most fit and correct code survives long after anyone really wishes it would.)

In high quality legacy nightmare code "why" is lost, unless someone wrote it down.

I've been on both sides of that mystery. "Why didn't they just do X?"

• Sometimes it was because X didn't exist yet, or wasn't matute enough.
• Sometimes it was because X is fundamentally the wrong solution, in a very subtle way.

There's two ways to know the difference:

• 1. Painful trial and error.
• 2. A comment (or document) answering "why".

I prefer the second way, but I happily charge more for the first way.