The PC (personal computer) itself doesn't settle down into a stable role, but ideas developed on the PC are usually spun-off into dedicated machines that are both cheaper and more reliable, as soon as those ideas stabilize. Take the MP3 player, the voice-mail machine, the personal video recorder, electronic book reader, or game console as examples. A PC can be all of these things, but we usually stop using it for those functions as soon as we buy the dedicated offspring. Everyone except hard-core gamers usually stop using their PC for games soon after they buy a Playstation or Nintendo, for instance, and the same thing happens for people who buy a digital answering machine and give-up the voice modem in their PC.
When PC functions don't migrate to specialized devices it's because the function hasn't been stable for long enough. Word processors, for example, have been embroiled in transitions for the past decade. In the early 90s they were transformed from simple book and letter-writing software to a much broader category called desktop publishing, which meant converting to WYSIWYG modes and adding better clip-art and typeface support. In the late 90s they all had to suddenly become web page and email authoring tools as well. It's precisely because the role of these programs hasn't been stable for long enough that dedicated word processing machines haven't been very popular. As soon as you pay for a typewriter with fancy graphical features, along comes the Internet, or along comes compound documents, and the market for fancy typewriters stays small because everybody wants to participate in the next round of experiments.
Whichever way you cut it, the PC remains as volatile as a mad scientist's laboratory. A global-sized laboratory, and one that's funded by the rats.