Concurrency was painful before actor programming. People did write code in single execution flow, occasionally communicating through memory with mutexes. Code was hard to scale to many cores and machines. Idea of isolated processes greatly simplifies use of multicore computers.
Collaboration is painful right now. We write code the same way we did when worked alone, occasionally merging changes into shared git repo. But what is the solution for collaboration?
We know how to scale computing to multiple cores, how do we scale work to multiple heads?