Rust began in the late 2000s and only in the last few years has started to reach a level of mainstream acceptance. Adoption for new programming languages takes significant time. Compiler development takes an awful lot of time. For Swift to be this mainstream after being introduced in 2014 is (IMHO) really quick.
As for Foundation, core frameworks almost never get rewritten and for good reason. It's almost never worth it. Best case? You get functional equivalence. Worst case? You introduce bugs into something that millions or even billions of devices depend on. Those bugfixes may take many years. Every now and again we see bugfixes that are 10-20+ years old in things like GCC or the Linux kernel.
Rewrites should never be undertaken lightly, particularly with less mature languages. Apple is (IMHO) being relatively aggressive with this.
As for Foundation, core frameworks almost never get rewritten and for good reason. It's almost never worth it. Best case? You get functional equivalence. Worst case? You introduce bugs into something that millions or even billions of devices depend on. Those bugfixes may take many years. Every now and again we see bugfixes that are 10-20+ years old in things like GCC or the Linux kernel.
Rewrites should never be undertaken lightly, particularly with less mature languages. Apple is (IMHO) being relatively aggressive with this.