@teajaygrey
I don't think Workbench would have needed any changes. It would still have been a fundamentally single-user UI. If there had been a multi-headed Amiga system for multiple users, each user would have run their own instance of Workbench.
Oh, interesting idea!
I did (co)SysOp BBSes on Amigas, Cit68k (Citadel variant) and CNET predominantly.
Both had multiple users, (CNET even had multi-user real time chat with multiple modems) yet they were abstracted away from Workbench and kept within the realms of their BBS paradigms.
I am heartened that Symbolics is apparently releasing Genera source (though my eyes kind of glazed over at the legalese of the application form and I have not yet applied) and I wonder if we'll ever see something similar with Amiga Workbench (as far as I know MorphOS and AROS are more like ReactOS to Windows; reinventions inspired by the original, yet not sharing any code with the inspiration).