Side note its not to "add to my argument". It's to improve the readability of the text for my sake and others
It's just as easy to keep up 3 DPS with a red mage as it is to keep up 3 healer with... any other role.
I agree that them being scared of relying on the healer is a problem but there are more interesting ways of making the healer role more integral to party composition in casual content. Even if they do not up the damage they can still add debuffs and mechanics that rely on the healer.
If you are taking shake it off and divine veil at face value, sure they have cure potency, but nobody is using those skills to heal the party. They use those skills to mitigate damage. Are you seriously expecting a tank to heal the party to max health? To even heal them efficiently at all?
Removing the self-sustain just isn't a solution when its an appeal to the tank class and very much needed in high-end content.
The healer role isn't even badly designed. The content you are doing has no need for a healer. There is no tank who is reviving the party, healing the party in a meaningful way, or even removing dangerous debuffs. No tanks are not doing your role for you. Dungeons specifically, have no reason to adhere to a strict 1 tank, 1 healer, 2 dps party compostion.