Having only 2 main damage buttons wouldn't be as bad a problem if healers had a reason to use their other GCDs. Incoming damage is too light and oGCD skills are free and plentiful. Bosses tickle the tank and will put out no damage for several seconds when casting abilities. If people die, it's usually not due to lack of healing but because they failed a mechanic.

When I used to raid hardcore in WoW, incoming damage high and frequent. As a tank, my number one priority was maximizing my survivability, not how much damage I could do. Doing mechanics was still a thing, but healers needed to constantly spam heals or people will die from other damage. Avoiding defensive stats should be something you do when you overgear an encounter, not when first stepping into a new tier. There's something wrong when healers rather meld Direct Hit over Determination when the DPS increase is extremely small and provides no healing benefit.

Incoming damage should be high enough that healers want Determination and Piety over Direct Hit, and tanks should want Tenacity, but this would require them to completely rethink how encounters should be designed and add a cost to and change many oGCD healing abilities. A big revamp like that would be something best done in a new expansion.