BrowserEngineKit is a thin wrapper over XPC and iOS' extension system. The system would be so much better to develop for if XPC was an open API, and JIT for isolated sub-processes was permitted without Apple's blessing.
* Messengers could have separate sub-processes for preprocessing untrusted inputs -- iMessage already does this, third-party messengers are single-process and cannot.
* Applications could isolate unstable components for better user experience and crash recovery.
* Emulators, e.g. for retro systems, would benefit from speedy emulation.
* WASM would become useful in iOS.
* Browser could use XPC without special-purpose API wrappers such as BrowserEngineKit.
But alas, all of this would make it easier to load code that runs at native speed into an iOS app after a store review happened, and as we all know that'll be the end of the world.