Earlier quoted context omitted.
A central key-value store that can be programmatically accessed to persist state across users, processes, and boots, that is also strictly typed and hierarchical is quite useful. I think it would actually be quite useful to have an /etc/conf virtual file system that could be programmatically accessed by user space processes and used as a "dumping ground" just like /etc already is.
I think the "central" part is where you lose me with this argument. What advantages does the registry have over application-specific KV stores... besides the potential to interfere with other applications and the OS itself?
There's a lot of value to having something like this "built in" where the plumbing isn't something you worry about. You don't have to worry about the state of your users' file systems (User deleted 'My Documents', User doesn't have a $HOME folder, you don't have write permissions for %AppData%, etc...).
I think if someone went off and redesigned a global KV store for an OS they'd probably require authentication tokens for mutations, so your app would only be able to communicate with the subtrees of the store that it has permissions to (but also, kind of like a file system?)