reddit/r/ffxiv
Poster: Lyritha
Original WHM/SCH pair worked well because 1) it was the only possible pair, which 2) made it easy to design encounters around it. You could have damage heavy enough that SCH's shields were necessary for survival, and you could have enough damage going out that WHM's heals were necessary for survival.
Enter Astrologian - in trying to preserve the heals vs shields dichotomy, they made AST capable of both, but not good enough at either (on release) that it could truly replace WHM or SCH. To add to the problem, Gordias was incredibly difficult and had very tight DPS and heal checks. AST also shipped with very bad MP management issues. This means AST was mostly shunned on release because its weaker heals (or shields) couldn't keep up. Midas was released and Diurnal AST gained some popularity, but it was still a difficult tier with punishing mechanics and very punishing heal/mitigation checks (A8S, for instance, wasn't cleared until people had enough HP through gear to survive mechanics) and WHM/SCH still had the edge. This means Nocturnal AST was largely ignored because you needed really solid, beefy mitigation to survive.
So... AST gradually got buffed throughout Heavensward so it could keep up with WHM and SCH and be a proper replacement for either. Enter Creator - the fights' difficulty was drastically reduced, along with several other QoL changes that made prog faster. AST also got buffed even further, and the buff was too heavy-handed (Balance's effectiveness was doubled, and shields were further buffed to the point they are today - the only SCH shield that is stronger than an AST shield is a crit Adloquium). This pushed WHM out of the meta because SCH still had plenty of free healing/mitigation and could offer more than WHM could, and AST's party buffing ability was off the charts ridiculous. Also, the existence of Indomitability covered for AST's lack of burst heals at the time. As a pair, AST/SCH could heal as much as a WHM could, while protecting and buffing the entire party (gentle reminder that Disable and traited Virus were on AST and SCH respectively) and spending fewer GCDs to do it.
This happened before any of the cross-role skill shuffle. AST/SCH was already meta then. This is when the meta truly shifted to what it is today - do the bulk of healing through oGCDs and have as much party utility as possible.
Stormblood came around everyone got access to Shroud of Saints (now Lucid) and a slightly watered down version of Divine Seal (Largesse). The devs made the (baffling) decision to reinforce WHM's identity as a "healy healer" (despite the meta having shifted away from GCD heals), and though they got Benison (which was hot garbage on release), it straight up can't compete with AST and SCH's mitigation. Adding insult to injury, SCH got a party buff (Stratagem) and AST got the incredibly ridiculous Earthly Star, closing the gap between WHM and AST for good - AST doesn't even need to rely on Indomitability anymore, as Earthly is straight up more powerful. Balance being nerfed back to its original state changed little.
The fights never got any harder, so GCD heals became less and less important, and party mitigation/buffs became more and more important. WHM got literally no new oGCD heals and almost every new thing it got relied on GCD heals. (Edit: Plenary is technically oGCD, but relies on GCDs to proc.)
And then AST got more buffs in Stormblood - Collective Unconscious applies instantly now, turning it into a simple oGCD you can weave, and Malefic's cast time was shortened by a second, making AST the only healer with zero weaving issues. The buffs WHM got (Lilies are slightly less garbage, Benison no longer needs lilies, Plenary Indulgence is more useful, Assize had a potency increase) never addressed the core issues, and other healers just kept getting better and better at what became the preferred healer playstyle.
And that is the story of how we reached the point where other healers can do everything WHM can that is actually necessary to clear content, while also being able to do things that WHM can't.