PLD/WAR and WHM/SCH were designed in ARR around these being the only tanks and only healers. They worked because there wasn't really any significant overlap.

When the devs introduced a third tank and a third healer, they should have narrowed the existing jobs so that they didn't cover as broad an area. They didn't. DRK was introduced as something in-between PLD and WAR. AST was introduced as something in-between WHM and SCH.

The end result over the past two expansions is that we've seen PLD and DRK trade-off with each other based on who is the better PLD, and WHM and AST trade-off with each other based on who is the better WHM. WAR and SCH dabble in a lot of different areas, such that their guaranteed spots have been relatively well insulated. They're also the reason why there wasn't really room for a third tank and healer.

Healers have another layer of problems with raid buffs (Balance, Strategem). I'm glad that tanks haven't really run into this issue yet, outside of the Slashing issue. Because stacking raid buffs don't have diminishing returns, it's pretty much mandatory for your job to bring one in order to stay competitive.

I'm hoping that the addition of a fourth tank and healer will force the devs to re-evaluate tanks and healers from the ground up. There isn't room for a fourth. You have to make room. The starting point for that is to look at how broad WAR and SCH's kits have become, and give them a more narrow focus, in keeping with everyone else.