Is it a design failure, or is it that poeple just do not want to tank/heal in the first place, no matter what they do.

If you, hypothetically created the perfect tank, would the DPS players start to play it? or would the fact that it is a tank, the role that they do not want to play, be enough to deter them? Same with healers.

Sure, there could be some design failure there, but how much has that really contributed overall to the state of tanking/healing? Maybe it is just encounter design not being hard enough for tanks/healers to necessitate the need for a simplified rotation. Having more things for a tank to look out for will help keep you distracted from the simple rotation, making things harder to heal, forcing healers to use GCD heals to heal everyone and making the decision of, should I re-apply the DoT or delay it to get the AoE heal out.

As this has probably alluded to, I don't necessarily think the roles are bad, it is just the encounter design doesn't support it.