> courtesy of modularization, separation of concerns, explicit APIs, and so on.

That's great, but a lot of that is knowledge you have to bring in. You need to be careful with the design of the API boundaries, the interfaces, etc.

Lots of people who are using AI to code projects... don't do that.

And once you get to the point that the AI is having trouble keeping up with the project... it's going to take a lot of work to discover what those module boundaries and interfaces should be.