The overhead will get absurd, you'll end up with 10x or more increase in engineers working on the system, all of them making slow progress while spending 90% time debugging, researching, or writing docs and holding meetings to work out this week's shared understanding of underlying domain semantics - but progress they will make, system will be kept running, and new features will be added.
If the system is valuable enough for the company, the economic math actually adds up. I've seen at least one such case personally - a system that survived decades and multiple attempts at redoing it from scratch, and keeps going strong, fueled by massive amount of people-hours spent on meetings.
Adding AI to the mix today mostly just shifts individual time balance towards more meetings (Amdahl's Law meets Parkinson's law). But ironically, the existence of such systems, and the points made in the article, actually reinforce the point of AI being key to, if not improving it, then at least keeping this going: it'll help shorten the time to re-establish consensus on current global semantics, and update code at scale to stay consistent.
[Infinite screaming]
In the end most challenges for a business holding them back to better code quality are organizational, not technical.
This is true. And I get sad every time it is used as an argument not to improve tooling. It feels like sort of a self-fulfilling prophecy: an organizational problem that prevents us from investing into technical improvements... is indeed an organizational problem.
In your example even as the interface for those products is unstable (UI that changes all the time, slightly broken API), those products are coded in a language like C++ or Java, which benefit from compiler error checking. The seams where it connects with other systems is where they're unstable. That's the point of this blog post.
Management and sales may not appreciate good software design and good code, the next developer that has to work on system will.
Example: Gambling is wrong but you can win big money.