In the long term it would possibly lead to people being a bit less 'all or nothing' on their opinion of heal GCDs. Pushing us back towards SB levels, where zero heal GCDs is the ideal goal, but not an attainable one for the later fights. As I previously linked, O12S was reduced down to 11 healing GCDs (and 4 Indoms). So if a group ends up with their best run being 12 GCDs and 5 Indoms, that's still a damn good run, and is more 'mentally acceptable' (at least, to me if I was in said group'. The fact we can get to zero GCDs wasted, means that even one GCD 'wasted' is a literal infinity% increase of 'wastefulness', so it leads to the 'all or nothing' of either, 'we have the perfect no-wasted-GCD-on-heals run' or 'we had to use a GCD on healing, wall it'.
In the short term, I'd expect people will refuse to adapt. The focus on DPS as the only metric that matters on the log page, as a healer, means that even if the amount of healing required increases, it'll lead to a fairly substantial number of players refusing to adapt, and instead forcing their cohealer into picking up the slack. Going from a Styx that requires the two healers to spend every OGCD they have but no GCDs each, to one that requires them to use 3 GCDs each (on top of the OGCDs), I would expect would instead see quite a few WHMs (it's always bloody WHMs) refusing to use Cure3 or anything, and so the SGE/SCH would be made to do all 6 of those required GCDs (Succor spam for example).
The only way around this (that I can come up with atm) that isn't to just play into it with systems that have, eg, doing damage building towards heal tools, and heal tools that are damage neutral, would be for FFLOGS to rework how the healer parse percentile is calculated. But that comes with it's own issues. For example, we could say 'it should factor in the HPS of the two healers, so that if you chad someone your parse is reduced' but me and my coheal friend did a run of... something this week, might have been the P11S reclear, either way, we had very equal HPS values and overheal %s, but one of us got a purple on HPS percentiles, and the other a grey, because damage prevented by mit is considered 'HPS', meaning shield healers naturally have massively inflated amounts of 'HPS' compared. So that'd potentially cause miscalculations on how much 'chadding' is actually going on
I'll just reiterate what I think: There should be GCD heals, sure. But they should be interesting to use. The ones we have now, should be the fallback option, not the 'pinnacle of gameplay for the role', because they aren't interesting. Lilies, for example, are interesting, because they are damage neutral, are gated by a CD, and provide new opportunities to the player (eg, extended movement lets you consider burning one, even as overheal, to remain damage neutral during a point where you cannot cast Glare). Medica is not, because you press it and it restores 400p of health, no CD, no player-count cap, just press it and it does the thing. At least Cure 3 has a range to consider, but they keep changing that to be more forgiving
Essentially, when we visualize it from the point of view of 'the goal is to do as much damage as possible', we can see that using a lossy GCD like Medica or Cure3 is like 'failing' in that goal, since actually hitting zero lossy-GCDs is an attainable goal. So, saying 'hey can you use Cure3 for this' is tantamount, to a parseminded player, to saying 'purposely fail your personal goal for the sake of these strangers', which, considering we might never see one another again (in PF), kinda does explain why someone would play more selfishly. So the way to address this, as SE, I guess would most likely be what they're doing, ironically: healing tools that are damage neutral, so that while you're doing your job, it's not at the cost of 'failing your personal goal'. Think about all those WHMs that are refusing to use Cure 3. They're all fine with using Plenary Rapture (which is just as strong), right? We're all fine with doing damage AND healing, it's when it's a choice between the two that it becomes a problem. So that's why I think SE should be doing things like that WHM gauge I suggested, make more unique ways to heal while remaining damage neutral. OGCD is one way, GCD with damage attached is another, GCD which provides a damage skill later is yet another. This is, oddly enough, another potential avenue for 'making healers feel distinct in gameplay'. For example, if WHM is all about GCD healing, then SE can build on that identity in the same vein as Lilies, and make it all about cashing in on massive bursts of damage, thanks to the refunds from those healing tools