The perfect UI for me is a command line. To hell with Windows Explorer, give me a DOS prompt any day. And with Linux, it's no contest for bash.
Command lines are so convenient. Not just in an OS either. Having used AutoCAD LT for a while now, I don't know what the heck I would do if they axed the command line. And that's just one example of how good command lines are. Can a GUI handle wildcards well? Didn't think so, unless it's *.* (Select all).
I highly doubt that command lines will ever be phased out on the higher level programs. No, XP is not a high level program.