Brittle by building ever more increasing numbers of abstractions rather that take a coherent view of some things. So over time you have these "seams" that the code base ends up looking like a punk motor cycle jacket.
In fact I think we're already at this scenario, but it hasn't had the predicted impact yet because the sort of people that realize we're here already overlaps perfectly with the sort already capable of developing their own software. When e.g. receptionists start tossing together their own productivity apps in a few minutes, then we will have entered into the next phase of the economy.
This isn't a matter of tooling through. Receptionists won't start making their own productivity apps because most people do not default to "I will make my own thing to solve this".
Maybe some receptionists will, but most won't, just like most people won't patch their own drywall even though the tools are available in every hardware store
Before computers, there were computers. [1] It's obvious in hindsight, but all of these complex numerical calculations we take for granted were often not done by the same people applying them in various ways. They'd hire computers instead, which were human calculators whose job it was to do the math.
The reason these jobs no longer exist is because calculators/computers not only solved these tasks, but completely trivialized the process to do so. We're entering into a world where many cognitive tasks, including software development, are going to be trivialized in a similar fashion - describe the problem, get an answer.
And I think this is going to do similar things to what calculators and computers did (or widespread literacy for a more historical example). The ability to e.g. write a program will no longer be a valuable skill in and of itself, but simply an expectation of anybody in any role where it might be desirable to do so.
People talk about AI solving coding, sure, to a large extent. The problems of design and ops completely confound the layperson. Just last year, a cousin mentioned they were using like lovable or something to build a webpage for their business idea. I offered to just throw it together for them, very simple crud. They gave me what they had on lovable, and they got no further than the landing page before they had no idea what to ask for, let alone how to ask for it. Prompts were literally "ok so now make the app" and the models like "well what about x y z"? You have to be technically inclined at the very least to get something remotely useful out of these things.
Edit to say that the best way such a person might use llms to make work easier is through whatever client they are provided, probably copilot, to do toil like data entry, scheduling, etc. but those lock you into that ecosystem and interface, and the solution:problem ratio is close to 1; one cannot extend or modularize such solutions because the concept isn't even in such a person's head. Perhaps this will be solved in time, but we are not there yet, and we're not close.
Granted, people would have to be able to accurately describe what they want to the computer
Many people cannot use a calculator to find the area of a triangle because they don't remember the formula for the area of a triangle, or figure out an angle because they don't remember trig
It's entirely possible that using AI to create software will run into the same problem. It won't be a rising tide lifting all boats, but only the top % of boats
That's still a pretty substantial problem if it is the case