You can imagine a function being unintentionally promoted into async-world for some kind of cross-cutting concern even if the data is stored in memory, e.g. if

You can imagine a function being unintentionally promoted into async-world for some kind of cross-cutting concern even if the data is stored in memory, e.g. if logging requires an async call, or if reporting metrics requires an async call. While I agree that in the general practice in JavaScript programming stops you from this kind of footgun, you can still be caught off-guard if you end up thinking that this programming style is immune to this kind of problem: locking is a big hammer with its own problems, but it will never cause data incorrectness.