Only Atlanta has failed,the other three are still in the league,so you can't say that they have failed yet
I don't know that still existing is the only barometer of success. All have serious attendance issues. Phoenix was getting run by the league for a stretch. Most of these previous expansion markets are far from thriving and the league wants to continue pushing the idea. I hope there is a way to resolve the unbalanced conferences, but I don't want it come through adding four additional teams. I'm not really even a fan of two more unless it involves relocation.