There have been many eras of computing as defined by the interaction between users and machine; their number is greatly under-appreciated. So let's take a walk through computing history. We'll set aside the batch computing era as prehistory since it predated interactivity.
The oldest era anyone here is likely to be familiar with is the shared computing era. This was an era when computers were big and expensive so many people were shackled to the one computer. Usually, even the terminals to talk to the one computer were shared. When the internet was new, shell accounts were the big thing.
After this era came the era of SLIP/PPP dialup which caused the rise of the graphical web browser. Before presentation was offloaded to home computers, it simply wasn't feasible to allow each user to run a graphical application on the central servers.
(Note that this new era was just a new paradigm and hardly an advance in every way. With the demise of shell accounts, it is no longer possible to search Usenet articles without downloading their contents to one's home computer. This was a significant problem before broadband.)
The third era we are in is simply the broadband era. Whether it is from the cable, phone or electric utility company, broadband provides enough network bandwidth that users make use of the network in a qualitatively different way (music and video swapping, and Voice over IP).
The fourth era just now beginning is one of not wireless but mobile networking. The hallmarks of this era are MP3 players, cell phones and to some extent PDAs. Electronic paper, if it ever actually gets built, would be symbolize the peak of this era.
The fifth era being planned in research labs like Xerox PARC's is one of UbiquitousComputing and will require that the current transition from personal computers back to social computers be complete.
What can we learn from this? That user's needs have changed so radically over FIVE (5) eras that retaining a dinosaur operating system from the dawn of the computer age is ludicrous.