While this may be a question that has been dated back to 3.4 Heavensward, I want to bring it up again to understand what exactly is going on with White Mage at this time. Considering that the other two healing counterparts have seen numerous buffs, we've only seen fixes to our problems. What I want to do is figure out for ourselves what a White Mage REALLY is in its current state.
To do that, I'm going to boil it down to what they have in their kit, as well as compare it to where the other two healers might have essentially copied from its kit.(Note: Stormblood saw a lot of changes, but one of the biggest issues was actions being taken from other jobs and simply spread to others)
White Mage dates back to the re-release of FFXIV, and was considered the 'main healer' in regards to raiding back in Coil. When Heavensward came out, White Mage got some MP tools to help it manage its MP better; however, before 3.4 we have to realize that AST was very broken to the point it wasn't really playable in Savage content due to its heavily lowered potencies at the start of Heavensward. This was fixed, and come Midas things started to look better in terms of healer balance sans SCH; however, when 3.4 came around they overtuned AST(similar to how they have now) to the point where it's a complete replacement for WHM in terms of capability and healing altogether.
When we got to Stormblood, Healer Balance felt a bit closer than before with how everything worked; main problem however was that Lilies was a very, VERY garbage Job Mechanic for a dated Job. SCH even has a more interesting mechanic with the Fairy than WHM does! AST itself has something that actually ENHANCES its play experience so that you aren't having to stare at what cards you've had for the past 2 minutes. What makes Lilies so bad is the fact that you can only proc off of Single Heals, compared to AoE Heals(Plenary Indulgence, will get onto that later).
The problem with Secret of the Lily is that even though you can gain Lilies through healing with Cure/Cure 2, it's mainly Cooldown Reduction. This encourages a bad playstyle in regards to what the Hall of the Novice actually has said to being able to weave in your own offensive abilities. The current reductions are 4% for one Lily, 10% for two, and 20% for three on use of an oGCD ability.
Continuing on, I'd also like to point out the things from ARR to Stormblood that caused our imbalance in the first place:
ARR
WHM - Had Stoneskin, worked very well with SCH(though there were only two healers back in the day) as a main healer, and was almost considered as a 'pure healer' during those times.
SCH - Supposed to be a co-healer, it was there for providing shields for the party and helping the WHM when pure healing wasn't enough.
HW
- Protect became shared between all healers, which is completely understandable.
WHM - Gained better MP Tools, as well as a free oGCD heal and stronger offensive capabilities(one being their AoE DoT)
SCH - Gained supremely better tools in regards to any healer in this expansion, getting the ability to AoE heal through Aetherflow, being able to spread their own Shields(including Crit Shields), and moreover having a way to convert their Shields into a pure heal.
AST - Introduced, but was very weak at the start due to improper potencies and MP management issues. Gained gradual buffs, and honestly NEVER got nerfed. Come 3.4 it became stronger than WHM due to how Diurnal Sect gained a +10% Healing buff. It also brought strong utilities in terms of its cards.
Present Day
- WHM's Stoneskin was removed, along with having Shroud of Saints Shared, Divine Seal(its main niche) being shared amongst the other healers as a +20% Healing Buff.
WHM - Gained Thin Air, allowing them to cast freely for the duration; Stoneskin replaced with Divine Benison, which was shared in the Lily Mechanic(and ate ALL your Lilies just for a 15% HP Shield that's on a 30s recast). Plenary Indulgence on the other hand was used with AoE Healing, supposedly needing to be used as a follow-up to AoE healing that was already done in case of incoming damage(usually wasted due to how strong WHM's AoE Heals can be along with co-healing being factored in).
SCH - Gained a Crit Buff, HP Insurance through Excog, and a very strong HoT to be its main gauge interaction with the Fairy.
AST - Gained a instant Heal through Lady of Crowns, as well as a way to deal 300 potency through Lord of Crowns through RNG. It also gained a 60s oGCD that has a heal STRONGER THAN CURE III(720 Potency Burst Heal, along with 200 Potency Damage). It even has an ability that actually helps execute its own Job Mechanic(Sleeve Draw).
Overall, I feel like WHM has not gained anything except MP Management Fixes and having parts of its toolkit shared with the other healers(Divine Seal, but I can agree with adding Shroud of Saints as a Role Action). Just what is a White Mage NOW compared to back then? Back then, it gave a particular job fantasy similar to that of an actual healer. As time has went on, it has felt like that has degraded further and further with AST getting what it needed until it got more than what was needed. If anything, I can see that WHM NO LONGER HAS AN IDENTITY IN THE GAME.
...And it'll continue to be this way unless SE decides to get their act together and actually look at the Healer Balance and what's causing such disruptions.