As for walled monopoly - what if Apple allowed Firefox with its free extension model - what argument would you come up then? One can easily use ublock origin with Firefox, a thing Apple fears quite a bit - its by far the best ad-blocking and to certain extent tracking technology out there currently. We all know here on HN that Apple is moving to marketing more and more (currently 4 billion/year for them and growing fast), so they will never allow this unless forced by law.
Which is one of those situations where users lose and corporation wins (unless you consider ads and tracking a good thing when Apple does it, but that's... illogical to be polite).
It can’t be that afraid, because you can use the Firefox version of uBlock Origin in the WebKit-based iOS browser Orion[0] right now.
[0]: https://apps.apple.com/us/app/orion-browser-by-kagi/id148449...
Extensions, as originally implemented, are a security nightmare. That's why every browser, including Firefox, is changing the way extensions work. Firefox is keeping blocking WebRequest specifically for ad blocking, but acknowledges the security risk. Apple and Chrome are removing it, which breaks uBlock Origin.
Ironically you don't need to look any further than extensions to see the impact of giving the entire web to Chrome. Firefox said they have to implement Manifest v3 because "support for MV3, by virtue of the combined share of Chromium-based browsers, will be a de facto standard for browser extensions in the foreseeable future." Imagine what Firefox would need to do if Chrome was the only other browser, with near-total market share.