Healers that don't want to DPS are like DPSes who refuse to move out of AoEs.

[Taika]

Absolutely nothing in this game requires you to keep your tank or other party members at 90% health apart from certain tank busters and big AoE hits in raids or extremes. You're pulling that number out of thin air and it's not based on anything but your imagination on how healers should be played in this game (which you don't even seem to have any personal experience from). As I said before:

A good healer minimises their healing while keeping all party members alive. When you use as little resources as possible on healing, while still keeping everyone safe (and yes, even 10% tank HP can be safe in some cases when you know nothing can crit them to death), you have the maximum amount of resources to spend on more effective contribution (DPS in the case of how this game works). If someone dies, then it's a mistake, of course, but the more skillful and effective the healer is the lower they can let people drop while still keeping everyone alive.

And as he said:

This isn't putting healing as a secondary perspective...
It's knowing the potency of our healing skill, mana management, knowing the approximate damages a tank is taking.

When you know what the damages mobs can do to the tank, you know what skill you have to use.
This isn't making dps or healing a priority. It is knowing to do both at the same time to be as much effective as possible as party member. That's the difference.