Apple is clearly moving towards a more secure set of operating systems and that starts by reducing the attack surface.
On macOS, Apple is deprecating kernel extensions; the boot partition on macOS Catalina is read-only so that it can’t be modified by malicious software: https://wccftech.com/macos-catalina-runs-on-dedicated-read-o...
Even Safari on iOS doesn’t support browser extensions.
>instead of simply allowing an open-market to thrive on iOS.
The App Store paid developers $34 billion in 2018; I would say the market is thriving: https://fortune.com/2019/01/28/apple-app-store-developer-ear...
>Safari/webkit, with a purposefully depleted feature set
Just checked html5test.com from my iPhone 7 running iOS 13.2.2; Safari scores 463 and Firefox 48 mobile scored 468 out of 555 points. And this is without enabling a bunch of experimental work-in-progress features: https://imgur.com/a/Y5NHwct
So there's no meaningful difference in capability of these browsers when it comes to features.
Of course, Safari is faster, requires less RAM and consumes less power, which should count for something on a mobile device.