I've worked on several thread-per-core systems, including embedded systems in which each thread owns a core and does not make system calls or context switch. You can do that, and it's ideal performance-wise, but it does not apply to multi-process systems -- how else are they to coordinate workflows, than through kernel-visible concurrency constructs: i.e., threads?

In language implementations with built-in user-mode schedulers like Go and Erlang, you get closer to this ideal for, as they will spin up a thread per core for CPU-intensive workloads. But even there, blocking system calls are still the norm (at least with Linux, which is only very recently moving toward a generic async system call system, with io_uring), requiring many more threads per core to saturate I/O workloads. And that still doesn't address the multi-process coordination problem.