When will Windows NT go away? When the equipment they are built into dies.
One of the perils of embedding PCs into mechanical equipment is that you usually build them with specific boxes, with specific drivers, and with specific OSes. My company has thousands of pieces of equipment around the world all running off of the same 300MHz Pentium PC that they were installed with from 1998-2002 and all running WinNT.
Fortunately the equipment in question is quickly becoming obsolete, so service is becoming less of a problem - but I still have to walk a new technician through finding the IP settings in NT that are somewhat different that its XP counterpart.
If you are thinking of embedding PCs in equipment, you had better commit to an OS (and its corresponding feature set) or commit to an upgrade schedule for the life of the equipment.
Chris