The software
wasn't unmaintainable. The PowerPoints etc were artefacts of a time when everyone involved understood some
implicit context, within which the documentation was clear (not cryptic) and current (not obsolete). The only traces of that context
we had, outside the documentation, were minor decisions made while writing the program: "what mindset makes this choice more likely?", "in what directions was this originally designed to extend?", etc.
Personally, I'm in the "you shouldn't leave vital context implicit" camp; but in this case, the software was originally written by "if I don't already have a doctorate, I need only request one" domain experts, and you would need an entire book to provide that context. We actually had a half-finished attempt – 12 names on the title page, a little over 200 pages long – and it helped, but chapter 3 was an introduction-for-people-who-already-know-the-topic (somehow more obscure than the context-free PowerPoints, though at least it helped us decode those), chapter 4 just had "TODO" on every chapter heading, and chapter 5 got almost to the bits we needed before trailing off with "TODO: this is hard to explain because" notes. (We're pretty sure they discussed this in more detail over email, but we didn't find it. Frankly, it's lucky we have the half-finished book at all.)
AI slop lacks this context. If the software had been written using genAI, there wouldn't have been the stylistic consistency to tell us we were on the right track. There wouldn't have been the conspicuous gap in naming, elevating "the current system didn't need that helper function, so they never wrote it" to a favoured hypothesis, allowing us to identify the only possible meaning of one of the words in chapter 3, and thereby learn why one of those rules we were investigating was chosen. (The helper function would've been meaningless at the time, although it does mean something in the context of a newer abstraction.) We wouldn't have been able to used a piece of debugging code from chapter 6 (modified to take advantage of the newer debug interface) to walk through the various data structures, guessing at which parts meant what using the abductive heuristic "we know it's designed deliberately, so any bits that appear redundant probably encode a meaning we don't yet understand".
I am very glad this system was written by humans. Sure, maybe the software would've been written faster (though I doubt it), but we wouldn't have been able to understand it after-the-fact. So we'd have had to throw it away, rediscover the basic principles, and then rewrite more-or-less the same software again – probably with errors. I would bet a large portion of my savings that that monstrosity is correct – that if it doesn't crash, it will produce the correct output – and I wouldn't be willing to bet that on anything we threw together as a replacement. (Yes, I want to rewrite the thing, but that's not a reasoned decision based on the software: it's a character trait.)