Nobody sensible is going to rewrite everything in Rust all at once. They'll start with new tools or things they needed to refactor anyway. Then use it where it's appropriate (it's certainly not a universally applicable language, the tradeoffs Rust makes are well known). Maybe someday AWS will have replaced all their Java with Rust, but I suspect some bits will stay Java for quite a long time, or migrate to Kotlin or some other garbage-collected language.