The problem “fiddle and see if Rome burns” is that you may have broken something that only runs once a year/decade and you won’t know if that process isn’t in the list of tests.

And by the time it breaks, will anyone remember the probable cause?

I agree, but what I'm talking about is distinct from a usual "process of understanding", because the goal is explicitly remain as ignorant as possible. In many cases I want to learn the exact minimum needed to safely make a change. What were they thinking? What motivated them? Don't know, don't care. I just want to clean up the mess and move on to something actually useful and pleasant.

As a practical example, last year I was supposed to figure out why a data collection system was not getting all the data we wanted. The person who worked on it was long gone, so I looked at the code. It was a bunch of stuff using Apache Beam. No tests, no docs. The original author was not a very experienced programmer. It jumbled together a variety of concerns. And after a day of trying to understand, it became obvious to me that some of the uses of Beam were unconventional. Plus Beam itself is its own little world.

The next day, I said "fuck that", and wrote a very simple Python requests-based collector. I pretty quickly had something that was both much simpler and much more effective at actually getting the data. And from there I went into the usual sort of feedback-driven discovery process to figure out what the current users actually needed, with zero regard for original intent of the system was in solving problems for people no longer present.

What was the thing doing? Why did it to it? How did it end up that way? All excellent mysteries that will remain mysteries, as I eventually deleted all the Beam-related code and removed its scheduled jobs. For me, this ignorance was truly bliss. And for others too in that the users got their needs solved with less work, and that developers had something much cleaner and clearer to work with in the future.