With everybody on Webkit, a vulnerability in it applies to every browser and nearly all apps on iOS. If other engines were allowed, a threat would only apply to apps using that engine.
On the flip side, keeping the engine centralized means that a single fix will close a hole in every app, keeping unmaintained apps safer.
I wonder if it would be possible to implement Firefox or Chromium in webassembly and run it on top of WebKit — it wouldn’t be as performant, but would be an extra layer of sandbox. You could theoretically do this in any app that uses WebKit too.
With everybody on Webkit, a vulnerability in it applies to every browser and nearly all apps on iOS. If other engines were allowed, a threat would only apply to apps using that engine.
On the flip side, keeping the engine centralized means that a single fix will close a hole in every app, keeping unmaintained apps safer.
I wonder if it would be possible to implement Firefox or Chromium in webassembly and run it on top of WebKit — it wouldn’t be as performant, but would be an extra layer of sandbox. You could theoretically do this in any app that uses WebKit too.