For example, my Pixel 4 just fell out of Google's 3-year support period, so I no longer get security updates. I would be completely happy to run LineageOS or some other alternative that would extend the life of the phone with regular updates, but I can't if I still want to be able to use Google Pay and other apps that require the phone to pass SafetyNet. Those sorts of apps are a part of my day-to-day, so being unable to use them would be a showstopper. (I've read various things about tricking apps into believing the phone passes SafetyNet, but none of the methods seem particularly reliable, and for every user who says it works, there's another person who couldn't get it to work.)
So sure, it is technically possible for me to treat the phone as "open" and run whatever OS image on it I want, but then I become restricted in other ways as to what I can do on it. Maybe you don't care about being able to pay for things using your phone (etc.); that's fine. But I do, and I consider it a critical feature these days.
They don't have to do that though.
Instead, they can wait for the EU laws to force Apple to allow them to do what they want, with their device that they own.
If you don't know by now that on iOS devices all the browsers are running the same engine under the hood, especially on a tech site like HN, then there isn't much hope for you.