I don't know how phone networks work in detail, but AT&T (among others) still offers dial-up Internet access years after DSL and cable broadband became available. EDGE is so much more widespread that I don't see it going anywhere for a long time.
Also, don't forget that a bunch of us will have EDGE-only connections for at least two years and many 3G iPhone users will only have EDGE available. 3G will take years to become as widespread as EDGE is. And to make another wired analogy, I know people who still can't get DSL because of the number of people on it in a location or they're too far out in the boonies.
So to answer thy question, no. EDGE isn't going anywhere. It probably won't go anywhere until we're long into the 4G discussion years from now.