Similarly, Apple has been slacking off on improving Webkit and making it as good as V8. By getting rid of Apple's monopoly on web engines, developers will be able to push websites to new levels that are currently hard to attain.
There's also some conflict of interest if say Chrome were to come to be the dominant browser on iOS since they also control Android. Would battery life, performance etc be prioritized as highly on iOS as their own platform?
I think it would be nice to be able to load apps from outside the app store especially on devices like the iPad Pro, but I think that's a separate issue from web standards. I think the only practical way to advance web standards is for the major browser vendors to agree and implement them, even if that sometimes takes a while.
But if another engine supports additional features that Safari doesn't, you could still offer those features to users of other browsers, i.e. you could do something like "install Chrome/Firefox to use our PWA".
> Would battery life, performance etc be prioritized as highly on iOS as their own platform?
Maybe not, but in that case people could switch back to Safari or another competitor like Firefox.