Copper wire is an outdated technology by at least 20-30 years and yet the telcos have not upgraded infrastructure. The public has said fuck it realizing that wireless offers greater mobility and there's rarely any point in having two phones.
In the foreseeable future, wireless Internet (via either IMT-* or 802.* methods) will make DSL obsolete as well, and the only reason for the telcos to continue to exist will be to maintain the lines
IFO keep a landline b/c it offers unlimited call minutes including long distance at all hours to an entire 4-person household at a price much less than equipping the whole household with cell phones and a comparable level of service. Not everyone is as cheap as me.
The only fear I have of people ditching the landline system is that, ultimately, right now, all our current wireless services interconnect through it. Your cell phone call ultimately feeds into a landline trunk. If that system gets neglected, your wireless service will degrade despite your belief that you are "wireless". I suppose eventually those systems will move off landline dependency, but knowing US wireless upgrade speed, it'll be slow and painful.
(And then we'll all get cancer from the massive amount of RF floating around everywhere, but that's not important.)