What’s really sad is that multi user system interrupts were long since a solved problem when Unix was developed. I don’t know why that existing body of knowledge wasn’t applied.
It is not as if the UNIX culture was to pay attention to best practices being done on other systems.
So it used very limited tools, like C with its very simple semantics and a preprocessor instead of a module system, very simple kernel mechanisms, etc.
Had AT&T been allowed to sell it, I bet we wouldn't have turned with it everywhere.
[1] https://www.cs.utexas.edu/users/EWD/transcriptions/EWD13xx/E...