You're right there a ton of things Safari does that others can't. So as you mentioned Apple would have to add all sorts of hooks to the OS and enhancements to the web view to allow people to use alternate brothers as the default even if they were still rendering with Safari internally, let alone some other engine.

Now would most users rather have the time spent on that, or improving Siri to give it more domains like the ones they added last year for third-party apps?

If the choice was between an improved Siri and the existing Chrome app as the default… Would that really be a smart trade off for their time?

( obviously Apple has the money to hire a lot more people, but let's face it that's not going to happen for this kind of thing)