They're great for browsers because the affordance fits: you have one active "browsing session", and then a bunch of inactive ones, and the inactive ones don't matter, and when you switch, you're asserting that the active one now doesn't matter.

But for productivity applications, tabs are just kind of silly.

In the OSX Finder, for example, if I have two windows open, I'm likely rearranging files between two-or-more folders—I need to see the state of both the source and the destination. You can drag-and-drop a file onto a tab, but you lose the visual cue as to where it's gone—it might have just ended up out of scroll-range in the current folder, or in some other random folder. To do the task quickly, I need to see both places: something done much better† by the window metaphor.

† (Though not perfectly: having to rearrange the windows to get the drag-sources and drop-targets all visible is a big hassle. Mac OS almost had something going for a while with MDI applications having their own virtual desktops, but they never far enough with it; if you could tell a given MDI application to auto-arrange/snap its document windows, people might actually start to want MDI interfaces again, rather than uberwindow "I know what's good for you" interfaces like FTP programs or Xcode.)

Then again, I'm also astounded that "tabs" aren't just a window manager feature (i.e. being able to group arbitrary windows into a single tabbed window, and noting when windows in a tab group spawn a new window and spawning it inside the tab group instead.) That would have cost too much GPU power decades ago, but it's costless with compositing window managers.

I'm trying to achieve this using i3 as window manager and a minimal browser like vimb which has no tabbing built-in. That way you can search for a browser page just like you would any other window.

(File management, on the other hand, is completely different; there, you're almost always going to want to have more than one thing visible and a windowed interface is better. That's why no mainstream file manager supports tabs OOTB.)

ZUIs are far too esoteric for the average WIMP paradigm nearly all computer users are familiar with. I wouldn't count on it anymore than I would on getting an Oberon/Cedar-like programmable text window interface.