This kind of AI system wouldn't just "displease the hardcore"; it would make WoW's combat easier, and therefore conquering it (within the player's subjective definition of "conquering" an essentially endless game) similarly easier. That means Blizzard would make less money out of it. Which is precisely why the difficulty will stay at the same level that it always is.
Even if it did, somehow, get implemented, it's not actually a great idea for MMO games in general. If you compensate for the damage done to tanks after destroying the healers, as you suggested, then that removes basically the only way to win against opponents in an MMO match.
In other MMOs like Guild Wars, destroying the healers is absolutely critical because they are essentially any party's link to an eternal supply of health - magic replenishes itself quickly, so the fact that it is used to restore HP makes it possible for one squad to take on 50 enemies in a single fight. And from what I know about WoW, the same rule applies there as well. Therefore, any AI compensation system like this is simply an artificial way of lowering difficulty, which ultimately removes the cardinal rule of MMO tactics (which is a constant simply because it is fun) - which is to take out the healer first, so that all others can fall.
That's only the tip of the iceberg. The damage to other tactical rules, and the inconsistency of gameplay that would result, would make PvE even more unstable than it usually is, making it impossible to know which strategy would work in any given situation. Worse, it would affect PvP because the expectations that apply to PvE would be so different that players would not adjust easily.
In short, it would be a disaster if this was implemented. The moral of the story is: do not take a piece of FPS pizza and put it in your MMO vegetable soup. The cheese will not curdle well.