I don't think there is much doubt AI can produce split out a lot of code, that mostly works. It's not too hard to imagine that one day an can AI produce so much code that it's considered a "large, complex project". A single mind dedicated to a task can do remarkable things, be it human or silicon. Another mind reading what they have done, and understanding it is another thing entirely.

All long term, large projects I'm familiar have been developed over a long time by many contributors, and as a consequence there has been far more reading and understanding going on than writing new code. This almost becomes self evident when you look at large open source projects, because the code quality is so high. Everything is split into modules a single mind can pick up relatively quickly and work on in isolation. Hell, even complier error messages become self explanatory essays over time.

Or to put it another way, no open source project is a ball of mud. Balls of mud can only be maintained by the person who wrote them, who get away with it because they have most of the details stored in their context window, courtesy of writing it. Balls of mud are common in proprietary code (I've worked on a few). They are produced by a single small group were paid to labour away for years at one task. And now if this post it to be believed, AI vibe coded projects are also a source of balls of mud. Given current AI's are notoriously bad at modifying even well structured projects, they won't be maintainable by anyone.