Cockpit is already a full replacement of the old Server Manager (NethGUI) and includes many more features.
It still marked a beta because it may have some bugs, but it will be soon marked as stable.
Some modules will not be ported to Cockpit, in particular those modules which are sort of deprecated (like the fax module). More info here: https://github.com/orgs/NethServer/projects/3#card-20260202
The manual already has a note for modules which will not be ported.
SOGo it’s a community project, everybody is welcome to port the user interface to Cockpit (I bet @stephdl already has some plans for it ).
Our goal is to support the old Server Manager for all NS 7 lifetime, but it will not receive new features. Maybe Cockpit will be default for NS 7.8, but you will still be able to install NethGUI.