[zzzcpan]

> but now you have to establish some form of IPC

Shared memory for threads is a form of IPC too, except one where it's very easy to make a mistake, introduce concurrency bugs.

> I also certainly cannot see any sensible argument for threading being harder than fork()

You should read a paper or two on concurrency bugs. Including on those using explicit but shared communication channels, like CSP does.

[the_why_of_y]

Concurrency bugs can be eliminated with state of the art static analysis (see Rust, Pony) - with the exception of deadlocks, which you can easily introduce with multiple processes as well.

http://blog.rust-lang.org/2015/04/10/Fearless-Concurrency.ht...