Well, of course, a lot had already been done by the time I got there. But what really struck me about CSP was how easy it was to reason about concurrent programs because synchronization and communication were the same thing. In my doctoral thesis I created a provably secure multi-user file systems which went from specification to CSP to occam.

It was not a fun time. In Java, for example, the concurrency & threading primitives were so low level it was almost impossible for anyone to use them and get it right. The concurrency package introduced in 2004 brought higher level concepts and mostly eliminated the need to risk the footguns present in the thread/runnable/synchronized constructs.

As far back at 1995 people were warning against using threads. See for example John Ousterhout's "Why Threads are a Bad Idea (for most purposes)" <https://blog.acolyer.org/2014/12/09/why-threads-are-a-bad-id...>