> Yes, and that's both memory and cpu usage that isn't needed

No, it literally is not. The "memory" is just entries in a page table in the kernel and MMU. It shouldn't worry you at all.

Nor is the CPU used by the kernel to manage those threads going to be necessarily less efficient than someone's handrolled async runtime. In fact given it gets more eyes... likely more.

The sole argument I can see is just avoiding a handful of syscalls and excessive crossing of the kernel<->userspace brain blood barrier too much.