I haven't had the chance to work on large codebases, but isn't it possible to somehow adapt the workflow of Working Effectively with Legacy Code, building islands of higher quality code, using the AI to help reconstruct developer intention and business rules and building seams and unit tests for the target modules?

AI doesn't necessarily have to increase your throughput, it can also serve as a flexible exploration and refactoring tool that will support either later hand crafter code or agentic implementation.