I've always seen it from the other side. It isn't that healers or tanks are so powerful that is the problem.
It's that 99.9% of the games mobs are designed to be "trash". They pose no threat at all... That's why tanks can tank 20 of them at a time.. because there designed to be "trash".
You can complete a lot of the games content without a healer at all. Can do a lot of it without a tank. Some of it you could probably do with just dps...
I think that highlights that it isn't that healers or tanks are too powerful that is true. Because even when removed entirely the content is still doable...
The problem is that virtually everything is deliberately designed to be weak and labelled as "trash" and simply doesn't pose a threat.
Even at 230 you can do sohmm hard with a tank and 3 dps. It's not powerful healers because you've basically removed them entirely and still able to clear it..
