There are 2 main types of players with very different playstyles.

The first type skips overland quests and zones and goes to Alik'r to power level to 50. This type of player has little or no interest in experiencing the full leveling experience and doesn't want to earn achievements on future characters so they find account wide achievements appealing.

The second type enjoys questing and exploring and clearing map objectives and gathering museum pieces. Account wide achievements completely robs them of this.

We've gone 8 years without account wide achievements. Not getting them now won't lose any players because nothing is being taken away. But adding this will lose a lot of players who will no longer be able to enjoy the vast world you created and be able to actually play the game.