I don't think that it's a foregone conclusion that Area51 generates niche sites, just because it generates niche site proposals. I for one believe that it's the big blockbuster stackoverflow-size sites that will actually make it to the end of the Area 51 process and will actually get created.

And, if I were a betting man, I wouldn't bet against StackOverflow's monthly 7 million users. In fact I would be betting on the statistics Stack Exchange site launching in the next couple of weeks and getting critical mass very very quickly.