I would still argue level three does not exist - future proofing is futile over the short or long term, and only maintainance is the solution. Good article otherwise though!
I think a more accurate statement of this sentiment is that investing time and effort into preventing future bugs and problems stops being worth it after a certain point (and perhaps earlier than many software engineers would think).
My favorite excerpt: The Shuttle software consists of ca. 420,000 lines. The total bug count hovers around 1. At one point around 1996, they built 11 versions of the code with a total of 17 bugs.
There are analogs in the mechanical world. Some of the most beloved motorcycles, bikes, cars, boats are easy to work on. Chevy made a straight six that people modified so heavily that people got multiples of the original horsepower out of it. It was simple, sturdy, easy to hang things off of and to modify.
The Open-Closed Principle tries to capture that idea in a sentence. But from all appearances the nuances of the concept seem to entirely escape people far too often.