Healers have been a pain in the butt to balance by SE because healing in this game is so overpowered on all levels that content where two healers is required really only requires the output of about 1.3-1.5 healers, meaning that the one who can DPS most effectively during the ludicrous amount of downtime is the one that's the most valued (in this case, SCH). Couple that with the fact that mitigation is REQUIRED during progression, and the fact that SCHs have a really diverse reactionary healing toolkit (Eos abilities, Indom, ETactics, Lustrate), and you have a job that shines in the healing department when it's needed, AND a job that can pretty seriously dish out the hurt when it's not.

The simplest solution IMO is to just seriously tone down how much our abilities heal for, so that 8-man content actually requires both healers working at pretty high steam to keep parties afloat, and to remove the necessity of SCH shields for raid content during progression - SCH shields should be a METHOD of healing (increasing eHP instead of recovering HP), but not something people need to survive stuff without wiping. Designing raids like this means that healers will ALWAYS be pigeon-holed into "reactive healer" or "mitigative healer", instead of having multiple styles of healing. A HoT-based healer would just be seen as a "reactive" healer that competes yet again with WHM/DiAST. A "time reversal" healer like Time Mage would probably fall under mitigative healing, and would have to have DPS capabilities basically like SCH in order to compete with SCH. Instead, if healing potencies were blanket nerfed and SCH shields WERE their primary method of handling damage, rather than the stupid amount of reactionary healing abilities they got added to their kit in 3.0, SCHs would have to focus more on shielding to prevent damage and less on firing off an Indom then going back to DPSing while the WHM carries the load.

What this would kind of mean: SCHs are still "mitigative" healers, but they're also not at the same time. Shields would be something they're constantly rotating, not just something they put up for tank or raid busters while using Embrace, Lustrate, and Indom a lot more often than that. They would lack tools to boost HP bars out of danger zones, so the whole job would become a LOT more focused on preventing those HP bars from lowering to begin with. It would also mean that other jobs can handle those lower bars or other jobs could be designed around preventing damage in other ways besides shielding (debuffs on mobs, split damage mechanics etc). I suppose the last example I listed could work even with our current meta, but my point is that when damage mitigation is REQUIRED, you're forced into taking a healer that can do that, even if your raid team wants to play a WHM and a Diurnal Astrologian. Other MMOs don't make their players really choose like that, in my experience.

It's definitely a hurdle for the devs to overcome, yeah. I don't envy them the task, but I also think that's why it's more crucial than ever that they put the brakes on developing new shinies to be broken at their launch and instead work on the core of their trinity gameplay - which means finding out what's working, and what isn't. I get that some people think they might not need to do that, or don't trust that they will, but try to look past not having a new toy to test drive this expansion towards the bigger picture down the line.