What good is a larger gene pool when genetic testing for hereditary disease is just around the corner?

There are several hereditary diseases in every breed of dogs. It's very unlikely that DNA tests will be developed for each and every one of these in the next few years. There simply aren't enough resources to do so. In a breed that has a narrow genetic base there are on average more dogs that have a hereditary disease than in breeds where the genetic base is broad. This is simply due to the increase in homozygosity: the genes that carry disease are more likely to be inherited from both parents.

Let's pretend that there would be gene tests for every concievable hereditary illness and that the tests would be cheap enough altogether for all breeders to have their dogs screened. If all the dogs in the breed would be tested and over half of them would prove to be carriers, everyone would quickly hanker back to a time when the gene pool was large enough so that new lines could be found that wouldn't be carriers of the same disease.

In short: DNA tests are of no joy if you don't have the genetic diversity to be able not to breed from affected dogs. If the majority of the population is related to each other there simply isn't any gene pool from which to draw new outcross dogs that aren't carriers of the same hereditary traits that are causing problems in the breed now.