Concurrency is a bug-spawning PITA. Managing access to shared resources is hard, and hard to test well. Any programming discipline which delegates responsibility for managing a large pool of shared objects to the user is a dead end.

The easiest way to achieve reliable concurrency is to minimize the number of shared resources. Using OS processes as your concurrency model achieves this without imposing any constraints on choice of programming language.

Unfortunately for those of us trying to write cross-platform compatible open source code, the menu of IPC techniques that work accross a wide breadth of operating systems is very limited. IMO, this is a problem which needs to be solved through OS innovation, not language innovation.