It just didn't work out that way back then. The reason why Heavensward didn't accomplish this at all was because damage output of a tank in Heavensward overlapped with DPS, and healer dps had a lower percentage raid contribution than it does now. I know that there were quite a few ongoing debates at the time along the lines of "If I use Shield Oath, then I'm helping my team dps," but again, that just wasn't the case. It was worthwhile to use a free mitigation ability to reduce your damage intake so that it may free up a healer global. It's wasn't worthwhile to throw away potency for it.