I don't think that anyone's claiming that Rust is objectively a simple language. Simpler than some other languages, certainly, but it's a medium-sized language at best.
Furthermore, unsafe code gives new users a firm boundary of complexity that can be ignored. New to Rust? Don't use the unsafe keyword. Using the unsafe keyword? Read the nomicon first. It's quite useful for onboarding to know that all the C-style UB shenanigans are behind a gate that can be ignored until you're comfortable with the rest of the language.
Furthermore, unsafe code gives new users a firm boundary of complexity that can be ignored. New to Rust? Don't use the unsafe keyword. Using the unsafe keyword? Read the nomicon first. It's quite useful for onboarding to know that all the C-style UB shenanigans are behind a gate that can be ignored until you're comfortable with the rest of the language.