Nah, it'd be limited by MP. The shields cost quite a bit of MP. Besides which, if it was the Job's functional rotation, this wouldn't be a problem. Let's be real, we already have effectively unlimited DPS neutral healing via all the oGCDs we have in the game. Everyone who brags about how they cleared an entire dungeon or fight with just oGCD heals? They just expressed they had, for that encounter's healing requirement, "unlimited DPS neutral healing". So this is already a thing. We might as well enshrine it and make it more interesting.
It really isn't. SGE is already over powered due to the oGCDs it already has. The healing costs MP, so it ultimately isn't free, and this would be more akin to SB SCH's Miasma 2 optimization. As for the optimized play argument - that sounds to me like an argument for reducing SGE's oGCD healing power (what it heals with now), not for limiting Toxicon.
Also keep in mind, for what it's worth, that you'll be reducing Kardia healing by using GCD heals still. While not huge, that is cutting the direct heal of Eu Prog/Diag by effectively 170 potency. And remember, unlike SCH, that's not pet potency.
Respectfully disagree. SGE already has very limited GCD healing options. Pneuma is once per 2 minutes, so the only other options it has are Cure 1, Adlo, Medica 1, and Succor. The only thing that would be reduced is that Cure 1 and Medica 1 would be less used in favor of Adlo and Succor...but this is ALREADY the way things are. How often are SGE's using Diagnosis right now? AT THE WORST, it simply enshrines what's already happening in the game right now. It doesn't make it any worse. Pneuma would still be worth using, and non-Eukrasia Diagnosis and Prognosis would still mostly be for niche emergency situations. So nothing changes. Note that the damage neutrality here would mean that Eukrasia Prognosis WOULD STILL BE USED for AOE situations, so no, it wouldn't force all AOE healing to the other healer in the party.
Right now, the situation is that SGE's try to avoid GCD healing ENTIRELY. How is "You'll be encouraged to only use 3 of your 5 GCD heals!" so much worse than "You're currently encouraged to use NONE of your GCD heals other than the 1 per 2 minute one"?