This thread raises an interesting question, but I dont think it is necessarily true. While it is true that fresh embarkations tend to be quite fatal, with a death rate far outpacing birth and survival rates, this doesnt necessarily spell death for the entire dwarven species, for the simple reason that we do not have a comprehensive understanding of how things work back at the established civilization.
The old mountainhomes must be heavily populated. Why? Because the king allows hundreds of immigrants to expatriate and run off to fledgeling fortresses(dangerous ones). If their loss would have impacted the needs of the population greatly, he would put a stop to all this immigration and lock down the mountainhomes. So, huge basic dwarven population in the mountainhome. QED.
Next, the new fortresses are dangerous, short of food and high in danger. Even elementary studies of animal population growth and reproductive health shows us that hardship and danger result in less breeding, while safety, security and reliable food results in more breeding. This is a simple question of energy consumption - when you have to devote more energy to survival, you have less to put into gestation and raising young.
So, if we can assume that the dwarven race, in some of its base ways, is fundamentally animalistic, then it stands to reason that fresh fortresses will not share the same breeding rates as the comfortable, secure mountainhomes. In fact, they are probably breeding like bearded rabbits back home.
Some dwarven hypothesists fiercely debate this point. They point out that human populations are quite different than animal populations, in that larger, established societies typically result in a greatly -reduced- birth rate as individuals aquire different pursuits than simple survival. To them, I simply recommend they go ahead and imply to the next dwarf they meet that they are similar to humans in any way, and see what happens.
So, to sum up: behind the scenes, there is an enormous population of horny dwarves back home, keeping up the numbers for the good of dwarfkind. No worries, friends!