New healer: Sage. Barrier healer.

[Rilifane]

Even then they are pointless because there is another condition that you did not mention, which is the current HP of the player about the be killed. If a restorative GCD will prevent their death because they don't have enough HP to survive an unavoidable, then it is better to use that instead of a shield. Right now, the only time a shield is viable is to prevent one shots at near or max HP.
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No, they would not be.
The current HP of a player literally does not matter as long as their actual HP + shield value exceed the amount the next damage would hit for. That's eHP.
If you bring them above the required threshhold by shielding them, they will survive just as much as if you head healed them above threshhold.
You could, theoretically, let a player sit at 1 HP for a whole encounter. As long as you slap enough shields on them that they don't break from damage, they will not die.

So if a barrier healer is designed in a way that the shields have higher potencies than their burst heal, you're better off shielding the next incoming damage than healing it at less efficiency. As long as everybody survives, it does not matter how you achieve it. Wether by shielding them or healing them.

Shields are valuable because they add HP to whatever HP the player(s) currently have, even if at max. They lose value because that is health that isn't needed to survive. This is what has to change if the devs want to invest on a barrier/pure healer set up.

The reason why shields lose value is because they have less potency than regens/ burst heals on ever single healer. Which makes healing the damage afetrwards more efficient as long as you survived it somehow.
Were it reversed and shields had more potency than burst heal, then it would be more efficient to shield people regardless of wether they would survive without that shield or not because it's less damage you have to heal back up before the next aoe. It really only comes down to potencies once you know people are going to survive the next hit.