A very interesting thought and in fact very, very much true in my own life. I can personally say I've stopped bothering about watches, calenders and a phone book (both physical and electronic) ever since I started carrying a mobile in my pocket. Even the home phone would have been tossed out, but better uses were found for it in terms of VOIP (both Vonage and MagicJack), however the home phone connection did take a walk though. With the advances in technology I'm also reachable at all times and not just to those who have my mobile number but even my email thanks to GPRS, EDGE, 3G and Wi-Fi.
A ran across a very interesting article at http://recombu.com/news/ten-things-mobiles-have-made-or-will-make-obsolete_M11191.html. Do take a look at it.