Andrei Alexandrescu wrote:
There's one final nail in the coffin. In wake of concurrency, de jure immutability becomes a necessity, not a useful and desirable de facto convention. Adopting the window dressing but not the essence of FP by a concurrent language evokes to me a scene in the Marx Brothers: an otherwise impeccably-dressed gentleman who forgot to put his pants on.
Language support is not strictly necessary to get the same effects as immutable types, as far as multithreading is concerned: small data can be copied, and large data can be made read-only by OS syscalls. This just had to be in the message passing library. (As a bonus, it isn't possible to subvert these mechanisms just by casting.)
