That's true, "on-demand" shields are pointless for the most part with the current encounter design... because damage is too sparse and low to warrant shields or damage is lethal/near lethal/provides vulnerability up debuff which eventually makes shields useless. There are a couple of places where you can stack AST shields with SCH shields to ignore the mechanic rather than deal with it, but as SE has shown with Deployment tactics nerf and introduced more lethal mechanics, they don't want you to stack shields to do this; they want you to resolve the mechanic properly. While Savage is difficult, it's not overly strict in the sense that you can't have leeway in using GCD shields unless you were aiming for a speedkill, but it wasn't always necessary either as not all parties had GCD shields. After all, we're currently in an expansion where any combination of healers are able to clear content, so the importance for clears were based on optimizing the similarly functioning tools all healers possessed at the high-end level (so the lv 80 capstone skills mainly covered any shielding issue).
It might seem like there's a lot of copium involved here (and there probably is), but it's not necessarily all copium. The current iteration of shield healers feel useless mainly because encounter design takes into account that not all combinations of healers will have on-demand shields. However if SE doubling down on making content clearable through 1 heal and 1 shield healer, it means they should be taking a specific stance on how they want these healers to individually function in a high-end raid, which implies they are tuning encounter design for this specific case and not tuning design for situations with WHM/Diurnal AST. Time has showed that SE does listen to feedback, even if they can't implement them all in the way players may want it. SE did hear the changes with wanting healers to have a DPS rotation, and we got SGE's GCD DPS focused gameplay as a result. I don't doubt they haven't heard encounter design being problematic with healer design and taken the necessary steps to minimize this issue either.
Also, 'shield healers' doesn't necessarily imply they only use raw HP buffer to block incoming damage. It's just a lot easier to differentiate them this way. 'Shield' healers also mean healers with high amount of preventative measures to deal with incoming damage, and on-demand shields are the easiest way to indicate this. Sure, AST and WHM has 'shields' but they're not in any way as numerous as the ones SCH and SGE has for preventing damage. What I mean by this is mitigation skills. SCH has a couple of mitigations: Fey Illumination + Sacred Soil + New AoE Mitigation. This is then in addition to their shield skills: Adloquium, Succor, Consolation x2/Seraphic Veil, Deployment Tactics.
SGE has: Lustrate with 10% dmg mitigation + Sacred Soil Equivalent + Pneuma (AoE mitigation/heal) for mitigation and Eucrasia Diagnosis + Eucrasia Prognosis for shields.
Even if most of their 'shield' abilities are oGCDs seem pretty marginal, they are generally low enough cd to use it often and readily available. There's a lot we don't know about sage, but based on what we have seen, they are definitely have a lot more 'shields' than AST or WHM who has 1-2 in their kit. Even then, we're not sure on the exact specifics since a lot of details are still up in the air after being changed to 'pure healers'. We don't even know if Collective Unconscious still has a mitigation component tied to it as Diurnal and Nocturnal sect is removed entirely, but the skills keep the initial 'Diurnal sect' flavor as a pure healer.
Until now, we haven't been able to see a real difference because encounter design never required SCH to be in the party for on-demand shielding (you can still run WHM with Diurnal AST and rely on Neutral sect). That may change now moving forward since it's very well possible to have a party with WHM/SGE, and we know from Live Letter that SGE currently only possess on-demand shields. In addition, they also went ahead and made Feint and Addle both work for both magic/physical mitigation as well, even if it's not as strong as the initial plan. To me, that speaks volumes to how important they're making 'mitigation' a factor in high-end content and may think we don't even have enough mitigation for certain boss fights...
though that can just be Quality of Life updates and I'm just copium. Take everything I say with some Copium.