My response literally pointed to any such change to make optional mitigation worthwhile without relying on gimmicks being largely impossible in isolation, and was specifically on when at-cost defensives would be optimal even when you are not "forced" (do or die) into them.
Tanks sustain's rDPS is the healer offensive value thereby afforded, because they are the only ones affected by its difference. You, as the tank, could exploit ("optional") percentile sustain around high incoming damage density in order to spare (the opportunity cost of) flat healer sustain for a net increase to party damage.
If the opportunities afforded through at-cost mitigation aren't worthwhile, then neither can that mitigation's costs ever be worthwhile outside of "consume would-be offensive resources, or die (which is a further DPS loss)." So yes, healer kits will be involved, too.
It's not merely "to make a point". That literally is how any optional defensive value works. If the fight is won by reducing enemy units' HP to 0, all other outputs besides damage will necessarily be weighed around how much damage their contribution allows for. Positioning enemies, preventing deaths, even safety measures to reduce cognitive load -- it's all ultimately about what wins that fight.So, to make a point, you adjust how healers work.
...Everything else is already accounted for. Tank sustain already includes all of their pre-alloted / "free" means of sustain. Any "optional"/"at-cost" sustain atop that is... atop what already exists. The only thing excluded from that picture is that actions which incur individual DPS loss can nonetheless create a party DPS increase, and such can still come in engaging and high-agency ways.With so many variables, you can see why changing one thing to make it better suit a particular narrative (whether intentional or not) can come across as a bit disingenuous.
It's not some rhetorical trick.
Nothing that is ultimately at net cost will ever be used, but a cost can be (more than) recouped either through its actor itself (spending resource on the mitigation ultimately refunds that resource or generates enough individual DPS, such as through damage nullified causing direct or indirect damage, to surpass direct offensive spenders) or through other actors (such as your healers not needing to spend as many GCDs of healing on you).
The first was already mentioned before the time of my post, so there was no reason for me to repeat it. Moreover, of the two, the latter is the more natural and intuitive (literally happens anyways if you just don't implode healer kits with an excess of free/pre-allocated healing, and doesn't rely on gimmicks that may take far more complex calculations to find the net result from), while the changes that would better allow it to be leveraged are ones already widely requested by healers.
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To answer your questions more precisely/individually:
Yes. Since that also be to the benefit of healing kits and healer gameplay in general.if we are tying more healing to GCDs, would this then be something that is more expected across all content, so that every GCD is not a damage GCD?
I've made no such suggestion.Why just change the one thing when there is so much more that needs to be considered.
Wrong, unless by "forced" you mean "whenever, under perfectly performed mathematical calculations, it would be an increase to party DPS."However, it still doesn't change the fact that the only time you would use it is when you are forced into it.
Tl;dr: Fundamentally, at-cost / "optional" sustain --especially where context-/timing-sensitive (e.g., percentile mitigation or recovery [see Death Strike] instead of just a flat shield or heal)-- is a wholly viable design in/for tank kits. The quality possible for that without relying on gimmicks (e.g., whereby your mitigation, itself, supplies damage directly or indirectly through your own actions), though, requires consideration of more than just the tank's own kit.