Remember that Swift collection types have the semantics of copy-on-write. Under the hood they actually only copy if the collection is non-uniquely referenced.
An Array<T> is a struct but uses a storage reference type under the covers. In theory the standard library is actually free to make common slices of arrays share storage so pure additions don't even require copying but I have no idea if it is actually implemented that way (neither String nor Array guarantee contiguous storage)
It definitely changes how a developer works in a language if they know they can depend on this kind of implementation. In Clojure, knowing up front that all data structures have shared persistence allows you to do things that you'd never consider in a language where this wasn't guaranteed.
I think Swift could greatly enhance its power in the functional paradigm if indeed it did what you describe, but also making this clearly known to everyone who uses it.
An Array<T> is a struct but uses a storage reference type under the covers. In theory the standard library is actually free to make common slices of arrays share storage so pure additions don't even require copying but I have no idea if it is actually implemented that way (neither String nor Array guarantee contiguous storage)