I had a similar issue with the Desktop folder. I would do this :
$ sudo rm -Rf ~/Desktop
$ sudo ln -s ~/Dropbox/Desktop ~/Desktop
And the symlink was replaced by a new folder at the next reboot.
As other users experimented (here and here), it turns out we can not lock a symlink anymore in macOS, the option is greyed out.
I found 2 solutions to this issue :
- Lock the parent folder (your home folder,
/Users/you/) but this will prevent you from adding/deleting files there (source)
- Disable completely iCloud Drive in
System Preferences > Apple ID (Keeping it enabled with "Desktop & Documents Folders" uncheck does not do the trick)