I'm not sure about these predictions of people leaving in droves for the 'next big thing' or what have you, not because I think that people wouldn't leave, but because I have my doubts that another Dwarf Fortress will actually emerge. Kieron Gillen put this particular chain of thought much better than I could over at Rock Paper Shotgun:
Dwarf Fortress sits at the furthest tip of the development brought to the gaming mainstream by Bullfrog’s Populous. The indirect-control management/construction game, with an eye on physical simulation rules. Except Dwarf Fortress is that taken to simply ludicrous degrees. I’ve been known to compare Toady’s opus to Proust’s Remembrance of Things Past, in terms of monomanical devotion to a singular vision informed pretty much entirely by your own interests. Dwarf Fortress is the designer as God, trying to create a machine which creates universes from a seed of a number at a button-press.
Dwarf Fortress is important because it’s fucking insane.
Source: http://www.rockpapershotgun.com/2011/02/18/the-greatest-pc-games-ever-5/I don't think this sort of game comes up very often, and I think the sheer amount of work to get even this far - a state where all but its most die hard fans will admit is in some places entirely lacking - suggests that holding one's breath for another Dwarf Fortress might be a dangerous occupation. I fully understand why people wish that an alternative would emerge, but I think that the frustrations with Toady sometimes ignore that his development priorities, e.g. absolutely everything else before a UI overhaul, are a result of him having the sort of personality that makes the game possible at all.
I'm not saying I'm necessarily right - someone could be working on a Dwarf Fortress beater even as I type this - I'm just adding my $0.02 on that particular topic.