It fluctuates heavily from expansion to expansion, probably because Blizzard doesn't know where they want healer dps to be either. Holy Paladin was meta for M+ back in BFA, no idea where it falls now since they removed their Glimmer of Light healing.
A really good Discipline Priest could actually top the dps back in Mists of Pandaria while also pumping out a ridiculous amount of healing, that is what I mean with "Blizzard doesn't know either".