The problem with quantum computing is that it requires problems be re-written specifically to the type of problem solving that quantum computers are capable of handling. Keep in mind, Toady hasn't multi-threaded DF yet because that requires re-writing the code to take advantage of multi-threading.
Also, quantum computers, as far as I have read, are only actually advantageous in certain types of problem solving. Granted, one of these happens to be something applicable to pathfinding, although again, the game would need a new pathfinding algorithm to do that, and simply switching DF to vector pathfinding would be more practical than requiring every DF player get some of the extremely limited access to a machine that requires being kept at near absolute zero at all times.
Quantum computing would also do nothing to actually increase the size of a fortress. The actual hard limitations, aside from the 16x16 map size, would be the 2GB memory limitation. Quantum computing at best might speed up some processes, but most players of "epic forts" are completely willing to play games at 0.25 FPS. (I've had games like that, as well - you just pause, input commands for an hour, then leave the game running while you go to bed, go to work/school, talk on the forums, and/or play a console game while checking back in every day or so to see what's happened.)