Web will finally become ChromeOS, takeover goal achieved.
Or other engines gain a foothold and web devs have to go back to standards.
Which other engines?
Why would they gain a foothold on iOS when they haven’t on desktop?
Extensions without going through the App Store is one
You don’t need a different engine for that; Orion can install extensions without going through the App Store today.
I think most people will continue using the default Safari browser.
That’s not how it played out on desktop and it isn’t how it will play out on mobile.
I use Chrome on Android because it's the default browser and I'm lazy, not because I actually like it. When a phone forces me to choose one I'm not very likely to choose Chrome. It's going to be the same for iOS users.