So i played Healer back in the Feast (Scholar) and achieved top 100 with it. While the Healer did have a huge impact, i do not think that it is accurate that only the healer mattered.

I had my fair share of matches where i outhealed the enemy healer tenfold but nothing died on the enemy side until i just ran out of mana. Tank Stuns and DPS CC mattered a lot especially later in the high tiers.

Also: being a Healer in the Feast was extremely stressful wich is probably the main reason the trinity got removed. Not to many had enough endurance to play and stick to the healer role until they got good enough.