I don't believe it's to do with the requirements. I think you'll still hit the same problems if those greenfield projects grow. It's still fundamentally about the code. I think you're missing the difference between a 10/100k+ lines of code professional software vs a quick 3k lines greenfield project.

In a few thousand lines of code you can get away with a massive amount of code bloat, quick hacks and inconsistent APIs. In a program that's anything more than a few thousand lines, you can't. It just becomes too confusing. You have to be deliberate. Code has to follow patterns so the cognitive load is lowered. Stuff has to be split up in a predictable manner.

And there's another problem, sensible and predictable maintenance. Changes and fixes have to be targeted and specific. They have to be written to avoid side-effect.

For organisation, it's been a huge effort on everyone's part these last 30 years to achieve that. Make code understandable, by organising it better. From one direction, languages have improved, with authors reducing boilerplate + cross-pollination of ideas between languages like anonymous methods. On the other, it's developers inventing + describing patterns or KISS or the single responsibility principle. Or even seemingly trivial things like choosing predictable folder structures and enforcing indentation rules[1]. I'm starting to feel that's often the main skill a senior dev brings to the table, organising code well.

Better code organization has made it possible for developers to make larger program. Code organisation is a need that becomes a big problem if you're not doing it well in large projects, but not really a problem if you're not doing it well in small projects.

And right now, AI isn't very good at code organisation. We might believe that you have to have a mental model of the whole program in your head, something an LLM is just not capable of right now. And I don't know if that's going to turn out to be a solvable problem as it seems like a huge context problem.

For maintenance, I'm not sure. AI seems pretty terrible at it. It often rewrites everything and throws the baby out with the bathwater. Again, it's a context problem.

Both could turn out to be easy to solve for this generation of AI, in the end.

[1] Younger programmers will not believe that even 15/20 years ago it was still a common problem that developers did not bother to indent their code consistently. In my first two jobs I'd regularly hit inconsistently indented code.

I personally find Claude Code has no real issues working and producing code in the 40k LoC Ruby on Rails repo I work in nor in the 45k LoC Elixir/Phoenix repo I work in. For the last few months I'd say 99% of all changes I do to both are purely via Claude Code, I almost never use my editor anymore at all. It's common things don't work on the first try or aren't exactly what I want but usually just giving an error to Claude or further instructions will fix it in an iteration or two.

I think the code organization isn't amazing, but it's fine and frankly not that much of a concern to me usually as I'm usually just reading diffs and not digging around in the code much myself.