Immutable by default doesn't mean you have to write your program as a giant copy-aon-write or log-structured architecture. You can still commit all your shared global program state behind a mutex/rwlock crimes when that makes sense. But you can contain write access to that data to the places where it's needed.

And at the function-local level move elision should turn copy-and-modify to in-place modification if the original is no longer used.