The real difference as I see it is that in the human maintained old style system, the human would take things slowly and ensure they understand any changes. Claude let loose will just say "yes, I know how to do this" and bork the system, because it doesn't have a concept of humbleness in the face of complexity!
I also don't even know if it's really true. Sure, maybe it's not coherent enough that a person understands it, but if an LLM wrote it, you can be ~99% certain (in my experience) that that LLM will understand it, and usually in less than the time it took to write it. And unlike humans, the LLMs don't die and take their knowledge with them, or change companies, or get ornery when you ask them to explain.
Also, in my experience, Claude doesn't really do what the parent is saying and dive head-first into complex problems and bork your code. It is fairly humble in the face of complexity. It constantly recommends breaking down larger problems into smaller ones that are more easy to grok.
But most importantly, anyone who's a decently skilled software engineer and who's using LLMs to write code is going to be thoughtful. Especially when they're writing code that's important. Unless you're an AI researcher pushing the bounds of what LLMs can do, which let's face it, the vast majority of programmers are not, it's almost always the case that you'll have your LLM writing intelligible code that you can understand and have it explain easily enough.
The idea that someone at say, Google, will be able to fit even half of the system architecture in their heads is not realistic. It also hinges on being able to find and understand the right slice of the system in any situation.
But we have all sorts of guardrails to make that easy - i.e logs, traces, etc
It is true that this gamble has existed in the form you mention for many years. Still, I sense something new here, both in scale and in the attitude of trust. Also, I find those massive, incomprehensible human-built systems frightening too.
So far I kind of feel like it nets out to the same bottleneck being good judgment from decision makers. And you might say, no, leadership will just put more and more pressure to add more complexity just because we can and it makes it appear that we are moving fast. But when it comes to real-world apps, there is a backstop which is that things that are too complex and illegible end up not being fit-for-purpose for human problems.
Claude Code and other general purpose AI tools are not like this, because they are literally trying to optimize it for every single kind of knowledge work with zero constraints on use case or target audience other than chasing the next juiciest one they can with essentially unlimited resource. 99% of software is not like this, and when the dust settles I suspect software won't be anywhere near as different as the doomers or the maxxers are predicting right now.