The big problem with native threads is its API. You have to make a callback aka function pointer. All of a sudden you have to keep track of parallel execution of these function pointers. This level of abstraction has no representation in code, only existing in the developer's mind. This doesn't scale.

There is an impedence mismatch between how code is written (sequential, top-to-bottom, left-to-right text) and how parallel code behaves. That's why people use idioms like actors or coroutines. I think to actually solve this you need a new format for writing code, like a graphical function call graph instead of a text editor.