Apparently these upgrades allowed Blizzard to keep pushing the maximum realm population up all this time, as more and more players joined the game. There are some relatively "low" population realms, of course, but only in comparison to the new much-increased maximum limit. As Drysc says, "aside from literally a handful of realms (I could count them on one hand), every other realm has a population that would have been considered high to overpopulated before the launch of Burning Crusade." Only recently have certain realms become truly crowded enough to merit free transfers to other realms.
People always like to talk about the eventual downfall of the strongest player in any arena, but the steady growth Drysc is talking about continues, it looks like WoW will be the biggest 800-pound gorilla in the MMO jungle for a good long time to come.