And the reason things function this way is because the dev team redesigned it this way. They claim that they don't expect healers to do damage in their design while also making it much harder to do good damage from ShB onwards.
On paper, it looks very easy. Spam your filler, keep your DoT up, use OGCDs to heal. But in practice, the moment something goes wrong and you have to use your GCD for anything other than your filler, you've already lost a chunk of damage.
That's what happens when they delete every other damage spell and shove all the potency into the filler spell, it makes it that much more punishing to drop a cast. The one creating this environment to be toxic in is the one that created this healer design in the first place.