Kinda disagree with the GCD being the problem. WOW has a GCD, too - nominally about 1.5 seconds, reduced by Haste (just like FF14's). Additionally, FF14 has OCD heals and instant heals. While these tend to cost resources (Aetherflow) or have CDs (Benediction), there are also spamable ones that do not (Regen). Indeed, you use an example of a WOW healer being able to throw out several HOTs, possibly even buffing them, but FF14 already allows this - a WHM can hit Lagress (oGCD) and then throw out a Renew every second or so.

Finally, this doesn't say anything about incoming damage. You could have a 10 sec GCD and have slower incoming damage and weaker healer heals. The game would be BORING, mind you, but the GCD itself being 2.5 secons (reduced by Spell Speed) isn't the problem there.