Since the NC is merely a display device, there is little that can become obsolete. An economy of scale . A multiuser system needs fewer megabytes of RAM and disk space,and fewer software licenses per user than the equivalent number of PCs. Greater security is built into Unix, and enforced by professional system administrators. Greater data integrity is achieved by automatic nightly backups.
Network computing is an old idea whose time has come again.
Early computers were centrally administered, serving numerous users via remote text-only terminals. X terminals are one of the many possible types of Network Computers, or NCs. It is typically the case that a great deal of the work done by a program involves re-drawing of client windows as you scroll, cut and paste, or open or close windows. This means that, when a change is supposed to be made on the screen, the server sends an X11 command to the NC, instructing it to do the computation necessary to, for example, move a window from one place to another. More users needed to be cost-effective.