But, I want that. With all the responsibilities that come with that.
Why can't I grant an app that permission? If Google discovers that an app with that permission is abusing what they are doing with that permission, then revoke their developer account! Delete the app from existing phones and inform the users that the developers could not be trusted! App store death penalty!
It's difficult to understand why there is any other reason other than maintaining their privleged position on the device to deny users this ability. Put a persistent notification in the status tray: "These apps have full access:", etc.
At the moment, you can do that, but not with an app hosted on the Play Store. I use a git client to sync my notes between my computers and my phone. But I had to get the app from FDroid, because it required the read all files permission to track changes.
Because you'd scream your head off like other HNers when a news article "100 million users private photos uploaded to Facebook and Genshin Impact!!!!" appears and would demand Google policing.
You can keep all your functionality, Nextcloud just needs to migrate to an API that gives YOU AS A USER control over what it can read instead of demanding blanket permission for everything.