Aka exactly the same use case as Art of War and Gravity no? Sage isn't really any different to the other healers here with only WHM being the odd one out on AoE due to needing 3 targets for Holy to be a gain.
Since AoEing has been bought it, it's an interesting example in how all single button spam isn't equal though. Wall to wall trash pulls deal spiky damage, healer AoEing (Mostly) requires some care with your positioning and just feels so much more engaging than standing 20 yalms away mindlessly smashing the glare button to dust.
One interesting avenue to explore would be simply going back to the days of both trash and bosses being able to crit. Couple that with healer DPS needing a little more positional thought (Bonus potency from the side or at a specific range anyone?) could really spice things up without the need for such a drastic overhaul IMO.
Eh, thing is, the vast majority of fight are simple fights like The Navel and ironically Tank damage relative to their HP pools and general durability is currently the lowest it's ever been in this game. Most dungeon bosses (With Dead Ends being a notable and enjoyable exception) are completely non threatening compared to the trash and even Extremes and Savage may well throw out big tank busters, but they are so infrequent now that you can cheese an unhealthy portion of them with holm/hallowed etc leaving enough CDs on the table to come close to 0 damaging the rest. Between said busters, the damage just isn't there anymore. The days of Twintania plummet stomping the tank every 30 seconds for the entire duration of the fight alongside constant auto attacks and death sentence are long gone. Bosses spend far too much time casting and not enough time actually stomping on the tanks.
Couple the sporadic bursts of damage with our over the top oGCD kits and you get the situation we are in now. If any healer is an outlier it's actually WHM now as we can GCD heal with Lilies in place of a glare at no cost.
As for adds in content outside of dungeons? They are hardly the norm, especially ones that can be stacked together. Most of the multi target bosses in Savage have caveats that either outright prevent aoeing (E6S Ifrit+Garuda) or had positioning requirements that limited AoE potential (E7S, P3S Fountains).
Pretty much this.