I would suggest indirection is one of the forms of abstraction? Making abstractions to “defer” architecture seems quite counter-intuitive to me, since the thing you’re deferring is implementation. But the architectural part — the abstraction — gets front-loaded. The idea that an abstraction can truly abstract the “how” of user persistence is very much the kind of fallacy I warned against when it comes to leaky abstractions. Do you want to use async methods for persistence or not? Do you want to persist is batches or not? How will you be notified of completion or errors? The answers depend very much on actual architectural implementation, and if you had created an upfront abstraction you may very well find it won’t suffice if any of these variables changes. So no, I don’t think that really defers architecture at all, it merely solidifies your current assumptions about how your architecture will probably look like in the future. If any of these assumptions turn out wrong or simply undesirable later, you’ve made things harder for yourself. So if that’s the risk anyway, it’s fine to just stick with a simple concrete solution when you can.

I do agree pure functions are harder to do in impure languages, but it’s not as bad as your example of C. Rust is very much an impure language, but its borrow-checker helps you to enforce purity constraints. In fact, using pure functions there is a great way to avoid getting into fights with the borrow-checker. If you use TypeScript you can use Immer to aid you, which is also included in the Redux Toolkit.