It’s not so much the apps as the system frameworks they’re built upon, which are still almost universally Obj-C (or some other C-family language). Even if every third-party or user-facing first-party app was 100% Swift, there’d still be massive wins from speeding up objc_msgSend.

(Not to mention that even pure-Swift apps use a lot of Obj-C-isms when touching said system frameworks, like the target/selector setup for most UI controls).