The mental model was required when your brain was the only chance to reason about changes, answer cross-cutting questions (architecture), and develop a visceral feel for the project, because that's what you needed to write high quality software

It's hard to let that go, but you already had to in larger human organizations/collaborations where you might be assigned work on systems you never/seldom touch, or coming back to a project you haven't touched in a long time.

You don't need a mental model when you can automate the reasoning and the benchmarks that vet the reasoning. Your mental model is better spent pondering->reconsidering high level things like invariants, and then automating the the proof and implementation of those decisions.

Consider how you can just get Claude to start a workflow of 15 Fable agents to fan out over your system looking for correction/simplification/perf opportunities before spawn another wave of agents to vet the list of findings. How much time and energy and studying of the code would it have taken you to build and vet the same list?