There are still things non-Safari browsers can't do, like send & receive SMS, set precise alarms/timers, handle SIP calls, customize your OS home screen, create overlays, etc. Even beyond Apple's agenda, there will always be things that are impractical for the web to do because it's cross-platform, and also dangerous for untrusted code to do. For example, basically all browsers have removed high-precision timing capabilities because it's too dangerous for untrusted code.

Also, web performance is still a concern on mobile devices which means it's still impractical as a native replacement for some use cases.