For anyone arguing WHM's deficiencies aren't a problem. I'll point to the singular healer comp that, while still viable, is severely limited: WHM and AST.

Now could it be Astro's kit contributing to the problem? To some extent, yes. Earthly Star, for example, trips over Assize as the two abilities often are up at comparable times and rarely is 1,120 healing potency ever needed in the spam of 20 seconds. Otherwise, Astro's healing kit cooperates fairly well with both Sage and Scholar. Of course, what actually keeps the job alive is raid buffs. Now on the flip side, White Mage and Scholar are the only two healing who force one another to GCD heal at least once because their kits can't synergize. They simply run out of oGCDs. I believe Scholar can mitigate this by dropping some Energy Drains but either way, it's a DPS loss. Sage works well but effectively carrying White Mage's deficiencies due to having so much free healing resources.

And it's that last point that causes problem. Astro simply can't carry White Mage. Even with the bevy of tools at its disposal, it still lacks mitigation options unique to shield healers. Okay, great! That's the whole point of having both a "Pure" and "Shield" healer.

Except Sage and Scholar synergize beautifully. In fact, if it weren't for Sage lacking a raid buff, this would be the meta comp without question. At least out of early prog. Mitigating damage as a whole is simply widely superior compared to healing it because all healers need to be capable of healing. Thus, you can't have either White Mage or Astro truly be a powerhouse in this category. And because they simply refuse to make outgoing damage demand shields, mitigation and raw healing, the former two dominate—in particular mitigation—once ilvl creep starts to trivialize everything. The long and short of it is White Mage needs to be carried for it to be both effective and efficient. Can it clear content? Absolutely. Can it do so without GCD healing? Yep. But every other healer can simply do it significantly better.

White Mage's "identity" is leeching off its co-healers better designed healing kits. Which wouldn't inherently be bad... if it had the damage potential to justify that. It doesn't.