The DVD format isn't going anywhere anytime soon. It was supposed to be replaced by Blu-ray but that simply hasn't happened because it's not practical to release low-resolution shows like DW in the high-def format (there is only one single story that can be released in high-def from the classic series and that's Spearhead from Space which was shot totally on film). DVD also has the edge because it never needs "firmware upgrades". The format is likely to continue exiting alongside Blu-ray for the forseeable future. Not everyone wants temporary digital files or to watch everything on their computer. So there will always be a market. Like books and CDs I do expect that it'll become a strictly mail-order market in the coming years, grant you, but they'll be out there. And when the "Cloud" crashes, and people start getting "usage-based surcharges" on their Internet bills (which are coming here in Canada), and all that, I'm confident physical media will out in the end.
A lot of folks, myself included, are following a statement made by Steve Roberts of the Restoration Team back in 2009 that the DVD line will end in November 2013. But since then I have heard nothing to suggest this will actually happen. Now, naturally it is likely that all complete stories will indeed be released by then, since there's only about 10 left and all but one or two have been scheduled for 2012. So that leaves the incomplete stories. And I still think there's a market for restoration via animation of missing episodes. We have Reign of Terror coming up, and it'll be dead easy to animate the one missing episode of The Tenth Planet so we have the first regeneration story. Couple the prospect of more stories being restored (you tell me there wouldn't be a big market for Marco Polo or a complete Daleks Master Plan), coupled with the "Revisitations" of older titles, and I could easily see the DVD range continuing into 2014 and beyond as long as there's a market for it.
And I still think it's only a matter of time before someone figures out a way to satisfactorily upgrade lower-resolution video and film elements, at which point we'll start seeing Blu-ray releases of classic-era stories.
Alex