But threads are still very expensive in terms of program understanding, and that matters much more than the runtime cost. A single-threaded program is much simpler to understand and show correct than a multi-threaded program. Threads are only free if you go to a full purely functional dataflow model, but then you can have neither resources nor other effects, so this is an option only for a small set of programs.
Believe me, that’s not where I am coming from 
I remember a long time ago the Oz language proposed that we should replace sequential programming with lots of lightweight threads. From what I could see, it did not work very well.