> Async may makes concurrent code easier to write, but also less simple to reason about as it grows. In a complex async codebase, I find it harder to reason about code flow and concurrency.

Good concurrency should make the code simpler to understand and reason about as it grows. Simply having process/service based encapsulation is a huge win. IMO this is a failing of the async/await abstraction, not concurrency itself.

> Good concurrency should make the code simpler to understand and reason about as it grows.

As an ideal.

But you can only go so simple once you have a piece of data that is read at the same time by many, while also maybe mutated by at least one. And as simple you can get version 1.0, as software grow, it gets new features which necessitate more interactions with that data, which makes synchronization more complex.