I've, uh, never quite seen why that would be the case.
There are, to our knowledge, two Dwarven kingdoms left. One in western Ferelden, one... somewhere very far away. Don't remember the location, but the analogy was New York and San Francisco. Clearly both are heavily isolated and under seige from a veritable sea of Darkspawn.
But... how do the dwarves keep the Darkspawn from coming through the various passages and Deep Road entrances across the continent? Or from digging new entrances? Or, well, what they do during just about every Blight?
It's not like the Darkspawn need to go through the Dwarves to hit the surface: if they did, there wouldn't really be Blights in the first place. The Dwarves are fighting to keep the Darkspawn at bay, but that doesn't really mean the Darkspawn are focused on them in return.
The first blight clearly showed the darkspwan were very focused on the dwarves. Over a matter of decades cost the dwarves almost all of their empire.
But after that, well the darkspawn are clearly following the song of the old gods, and when she was awakened, the loss of that song is what drove the Mother mad, along with her disciples. Duncan says to a Warden who hasn't been recruited yet that Darkspawn still sometimes make to the surface in small raiding parties. The Anderfells also seem to suffer a huge darkspawn problem according to all sources.
Also, according to all sources, when it isn't a blight the darkspawn are constantly advancing and are regularly at the gates of Orzammar itself. Heck, even Felsi at Lake Calanhad is completely calm when asked if she's worried about a potential darkspawn attack. She says that "I'm from Orzammar, a darkspawn attack is like a spring breeze." Or something like that.
Heck, my Aeducan Warden told Duncan that it was about time the Darkspawn left the gates of Orzammar to bother the surface for a change so they (the dwarves) get some room to breathe.
If the dwarves are wiped out, maybe the darkspawn wouldn't flood the surface, but the fact that they are always advancing except when an archdemon is leading them makes me think that they would keep pressing the only they can. Up to the surface. Or follow the song to find an archdemon. One of the two. Darkspawn as a rule aren't organized or great strategists, but are a horde, so you can't guess at motivations beyond base instinct.
Besides, even if the dwarves are wiped out and the darkspawn don't attack the surface, it would only be a matter of time when templars would cease to exist as there wouldn't be any lyrium to give them and make them resistant to magic or give them templar powers to negate magic. Nor would mages have the lyrium to do a Harrowing, enhance their powers, or do any magical rituals. The mages, unlike templars, can replace lyrium with blood magic, however, but that's something no one really wants.