Can’t say I disagree with you. A multi-threaded, memory safe cat sounds like a worthwhile future project for the Rust community.
I didn’t understand the need for a multi-threaded version of cat. File reading/writing is a sequential thing, right?
AFAIK operating systems delegate reads/writes to their IO subsystem. This subsystem can optimize the IO operations by scheduling them in an order that differs from when they arrived to the IO queue and this improves performance in some cases. Instead of FIFO the subsystem operates on the segment of your storage device that is closest to its current location. Modern IO subsystems are both thread-safe and smarter than the average programmer.
A big reason is that they are usually multi-threaded, while the C counterparts are not.
Writing (memory)safe and maintainable parallel code is much, much easier in Rust than in C.