The addition of death knights to the game is in many ways like the addition of blood elves and draenei in The Burning Crusade. Many people made new characters just to see the new zones, but many others wanted to level their mains through Outland first. Many of those players who tried out the new races only played up to a certain point and then stopped to go back to their main characters. We never saw a flood of draenei and blood elves outnumbering all other races of Azeroth, and for the same reason we will not see a flood of new death knights. There are different things to do in the expansion, and different people make different choices about which to do first. There may be a contstant stream of new death knights, maybe even a river sometimes, but death knights will just feel like the newest kid in the WoW class, not a plague of locusts infesting the entire town.
There's also a huge difference between trying out a death knight, and choosing one as your new main character. Wherever death knights start out in the world may be a crowded area for a while, but most players won't ever level them out of that starting zone. Unlike the Jedi in Star Wars, death knights are only one of many types of characters in Warcraft. Besides, the death knight play style and thematic mood simply isn't going to appeal to everyone, in the same way that most WoW players today do not play warlocks, notwithstanding the fact that warlocks are undoubtedly a powerful class. Most players prefer to do healing, shapeshifting, stealthing, ranged shots, totems, or any number of other abilities that death knights will never have, and they will stay with their favorite classes and play styles. Some players, like myself, probably just won't like their armor decorated with skulls all the time.