In defense, I would argue that many modern language design advancements DO relate to multithreading in some way. Rust’s borrow checker and linear/affine types can help with thread safety; OCaml is using algebraic effects to implement multi core behavior; GPU-related experiments (such as Formality [1]) are all about enabling more scalable and reliable parallelism.

That said, I don’t think that multithreading is the only interesting or worthwhile part of programming language theory these days. However, I think it is extremely fertile for innovation while also being somewhat isomorphic with other interesting correctness/performance/ergonomics problems.

[1]: https://github.com/moonad/Formality