Ok here are my personal thoughts on the subject:
-First we ALL know that new things are interesting. It is VERY probable that in the first few days all high level players will rush to do the quest and try the DK first thing after the expansion. HOWEVER, this doest NOT mean that they will stay that way. Why? Simple, people chose to play their class because they like the mechanics. So as in the previous example, I say that if someone chose to play a holy priest to 70, it wasnt because they love tanking.
-BUT, considering that the DK seems as a class with the following attributes:
DPS: Good, can duel-wield, offensive spells
Healing: Probably only self by leeching, and not much? Not sure
Tanking: Good, but cannot use shields, probably depending on dmg output or skeleton minions.
It is very POSSIBLE that the following classes may stick with it, as they have similar mechanics. They are in order of possibility, the first being the most likely to stick with the class:
-Retribution Pallies: Want DPS, some self-supportive healing, able to fight a decent number of mobs without dying. PLUS offensive spells instead of many holy spells made for healing are MUCH better fit for the DPS job. Classes are supposed to be VERY similar, although ironically are opposites.
-Fury Warriors: Want DPS, dont care about healing, want surviveability (not dying as fast), rune system aparrently SIMILAR to rage system (runes regenerate over time, even while in combat, at same rate)
-Combat Warriors: Same as Fury, except a balance in DPS and Tanking capabilities
-Demonology Warlocks: Both classes similar in spell types, tanking ability can replace pets to a certain extent, DPS oriented classes, Dark spells tend to include in MANY cases DoT spells.
-Feral Druids: A mixture of ret pally and fury wars' reasons
-Combat Rogues: Same as Combat Warriors
So, in other words, no current tanks are likely to take up the DK as no shield or some other bonus apart from dmg induced threat does not apply to their current gameplay, and healer are definate no-noes.
In conclusion, NO there wont be a healer wipe from WoW, because healers actually LIKE healing. Second, the DK style of play MAY OR MAY NOT be very similar to a number of classes, causing them to MAYBE turn towards the DK for a longer while or even permanently. I just listed the ones that have more REASONS to do so. Again, no one knos if all priests are actually making a boycott with blizzard to sabbotage WoW, roll DKs and rule the world of teddie bears. Again, it varies, so NOTHING is certain.