Raw root access isn't what I'd want apps to have.. it's that the Android permission system deliberately limits what the user can consent to, the rest is for "system apps" and to install those you need to unlock bootloader and start the whole "journey" while saying goodbye to banking apps.
Implementing a more flexible permission model + sandbox would probably involve too much work for them.
Hopefully AVF might make things a little better if we'd be able to run Android VMs on Android - so you'd be able to run a rooted VM inside GrapheneOS.. but this depends on Google keeping Android open source, yet QPR1 was not released.
I agree that a powerful permission model is a great feature. But that doesn't obsolete the option to have the "root permission" that you can give when required. Sure, for my specific gripe a "full filesystem access" permission would be sufficient and better. But there are going to be other use cases that require other permissions. So it is always going to be useful to have that backup root permission that you can assign to very specific apps when required.