This level of security exists on open as well as closed platforms, the problem is the closed platforms not allowing you to do things that aren't giving your password away (like installing fdroid or using beeper easily). I just have a hard time believing this is superior in any way.

I think you're confused.

I you run GrapheneOS, it is an open source platform built on top of AOSP (the Android Open Source Project). Part of the security model is that you don't run as root. I am an advanced user and I don't want to run as root on my phone, I am happy with GrapheneOS as it is distributed.

Now if you want to be root, you can install an OS that allows you to be root. Just like I unlocked my bootloader, installed GrapheneOS and relocked my bootloader, you can do that and install whatever you please. I will keep using GrapheneOS because that is the most secure OS I can find for my phone.

The problem, IMO, is not that "some OS are opinionated and don't give you root access while other OSes do give you root access". The problem is that on many phones, you are not free to install the goddam OS you want (e.g. because you can't unlock or relock the bootloader).