I've been working on a project that's over 150k loc of Rust at this point. https://dirge-code.github.io

You absolutely can have the LLM write maintainable code. A few tricks I use are to ask it to plan out features in phases, and then do a branch and a PR for each focused piece of work. It makes it a lot easier to review and understand what's happening.

I also ended up making a tool which lets the LLM get a high level perspective of the codebase, and then see parts that are structurally gnarly. I've been using it to do refactors and clean things up periodically. It helped a lot with keeping the architecture clean.

https://github.com/yogthos/wavescope-mcp

Adding features and evolving the codebase has not been a problem even at this scale.