In the idea of a paperless office, the fundamental object will be a document, and it might as well exhibit as (virtual) PDF as exhibit as a window. When you wanted to call up a document this PDF would become real, either as print or (possibly interactive) display. This seems oddly SmallTalky but with a modification of the idea of document.

I have to come up with an inventory management system for lots a parts from several manufacturers. I wanted to use SmallTalk for this because I sense that the difficult problem will be defining how the inventory flows (what you call business rules). But I don't want the users to see any of the guts and I want to get away from "everything in a web browser". So I guess I want to detach from windows an have explicit, formatted documents displayed.