Last thing first: Which is why I continually propose that there should be 4 different levels of play for the 4 healers so everyone has at least one they enjoy.
To the 1 and 2: Note that 2 is based on 1. That was kind of my point. You can't treat it in isolate. Based on 1, noting that 2 means FFXIV is comparable to other MMOs in healing from that time makes sense. And to be fair, it's not like it was ever hyper DPS intensive - WHM in SB only had one more single target DoT (Aero and Aero 2, Aero 3 was for AOE, not single target). If we were using a substantial number of GCD heals, then having few GCD damage spells wouldn't be noticeable since you don't sit there pressing Glare over and over again very often.
As to your note about what damage would have to be - it's less damage would need to be ridiculous and more (a) oGCD heals need to be toned down AND (b) healing needs to be more continuous/sustained. The current healing model is that healing generally comes in bursts, then periods where no healing is needed (absent mistakes). What this means is that the bursts can never exceed party health pools (at least with mitigation), as they would just KO everyone. This then allows one or two oGCDs to be used to get the party back to survivable health before the next damage spike. If the oGCD heals were weaker, this would require GCD heals (you simply wouldn't have enough oGCDs for them all, and so oGCDs would assist, not replace, GCD heals), or alternatively, if damage was more continuous but SMALLER, you'd have to use GCD heals since most oGCD heals are spikes all at once with only a few being HoTs. That is, if you take 50% damage then no damage for 30 seconds, an Assize will do the job. But if you took 5% damage every other GCD, you'd run out of oGCDs 30-40 seconds into the fight and then have to cast GCDs - more realistically, you'd space out oGCDs or hold them for bigger hits and use things like Medica 2 augmented with Solace/Medica to deal with the tick damage and the oGCDs for larger attacks.
Point is - at the very least - the current encounter design is garbage for healing.
...in one way or another, I assume we can agree on that.
EDIT: I should also note that FFXIV HAS HAD fights like this at various points in its history, to mixed effects. The king Behemoth in Labyrinth of the Ancients, for example has a constant electric damage on all party members. Back when synced party members didn't outheal it basically just on natural regeneration, it would require things like Medica 2 (since WHM had no AOE oGCD heal at the time) and the like to keep the party from being whittled down to death.