Arcane crest doesn't make healer buttons obsolete. It's used for raidwides or stack marker mechanics but it can't heal a tankbuster. If it was enough to make healers obsolete, then we wouldn't need healers for the Final Boss of Endwalker. Personally, I rather keep Arcane Crest as it is. Mantra has its own purpose when paired with healers - since it can be stacked with all sources of healing. Different strokes for different purposes. Mantra is more flexible even if it's weaker. Arcane Crest is more focused in its effect, and intentionally trying to use it on a mechanic will only cause the Reaper to get a vulnerability up debuff, which defeats the purpose of an oGCD heal.