Rust requires discipline too. I can go around using Arc, Rc and .clone() everywhere without upsetting the borrow checker, I can use let mut a bunch and pretend if, match, etc. aren't expressions. This results in worse code, and Rust didn't stop me.

The borrow checker prevents a set of errors from being possible, but it doesn't prevent bad code from being written.

It would be easier for you to argument that the user is expected to have discipline to NOT use "unsafe" keyword in all functions.

Because a lot of mechanisms actually still have guards in runtime. And using .clone() on Rc/Arc is actually the idiomatic/preferred way of evading the borrow checker if we can't design the data structure in a different way.

It's a big difference between cases when you need to spend brain energy to find ways to "out-smart" the compiler, and spend brain energy to "fit into the proper set of assumptions" of a programming language.