Large part of why it's essential is because incoming damage is a joke 95% of the time

Tanks can be in dps stance and still take so little damage they don't really need healing outside of a fairies embrace or couple of regen ticks from a Whm.

The game is built almost entirely around dps which is why plds have had so many problems. Because it's a defensive tank in a game where defence is mostly worthless. And why most of the time over half of a Paladins toolkit is also useless.

So unless stuff starts hitting hard enough that healers actually have to heal and Tanks actually have to tank cleric will remain almost essential