Well I have to say that Seattle represents the opposite trend. When I first arrived about twenty years ago, fresh from Saotome Sensei's DC dojo, there were three dojos that I know of.
Today there are at least fifteen in the immediate metro area. Pam Cooper, Joanne Veneziano, Kimberly Richardson, Fran Kialpha, and myself all trained under Mary Heiny Sensei and now have our own places. Martha Levenson, Lee Crawford and Bill Gray were all at my school early on and now have their own schools. Bookman Sensei came back to the states from Japan and set up shop here... His dojo has had offshoots. MacEwen Sensei from New Jersey has a student here named David Morris. There are some Seidokan folks whom no one seems to have met... I am sure that I am forgetting some others.
I think that the biggest obstacle to more dojos opening is the maturity required now. Most of us opened our dojos when we were Sandan. In those days there simply weren't folks who were senior to us in the area. We had between ten and fifteen years experience.
Now most of these folks are Fifth Dan and even Sixth Dan. We have between twenty five and thirty years of experience, most have been teaching Aikido in one form or another for twenty years or more.
So now if you are looking to start a dojo and have any credibility you'd need to have a lot more experience than what was required fifteen years ago. Not that we need more dojos here. Anyone interested in Aikido should be able to find a dojo that is compatible with their preferences within a twenty minute drive.
But there is also the dedication factor. When I started with Saotome Sensei he flat out stated that he was training professionals. The people training in the dojo were all folks who saw themselves as Aikidoka, they had jobs that supported their training. It was the same at Mary Heiny Sensei's dojo as well, which is why so many of her students now have their own schools.
These days I have to say that it is much harder to find people who want to go the distance and make the commitment to become Aikido teachers. I have a very healthy dojo size wise but I have only three people who I think are going to end up teaching. The rest are regular, middle class folks with families and demanding careers and lots of other priorities. Maybe the times have really changed and the generation that came out of the seventies looking for alternative everything had been replaced with people who are generaly content with the way things are. Anyway, I find it difficult to find the kinds of students that used to populate the dojos in which I trained years ago.