While no one here expects a player who has disabilities to perform in ways in which they physically or mentally cannot (this is mentioned in just about every DPS-debate thread when someone brings up a disability), I don't think that counts for the majority of idle players you see in dungeons. That being said, unless a person says "I have such-and-such disability; and I physically cannot do this and/or that", the group has no way of knowing. So unless people are wanting us to assume that everyone who doesn't DPS as a healer or doesn't AOE as a DPS has a disability, what are we supposed to assume?
"Casual" content doesn't mean people can be inherently lazy, disabilities aside. If a person has the ability to carry their weight, they should do that, rather than expect everyone else to pick up the slack for them. Anecdotal, but I've noticed when wipes occur, a lot of the people who say something negative about it are the same people who aren't pulling their weight, be it a tank not properly holding aggro, a healer staring off into space as the tank goes down (actually saw that just yesterday in Kugane Castle--3 wipes on the pull before Yojimbo and the healer didn't try to salvage any of them, rather instead blaming the tank who, I guess, is expected to heal themselves now, and then they started blaming the DPS for not killing the bomb add, which died almost immediately and wasn't what caused the wipe; the healer just wasn't healing the tank, and they weren't DPSing), or DPS single-targeting a pack of 10 mobs.
The main argument is, and has always been, that everyone should equally contribute to any and all content. Not one person doing the minimum and expecting the rest to compensate for them.
That is a very big generalization you're making there. And, honestly, I find it incredibly incorrect. Threads here alone suggest the opposite: content is too faceroll, and people still cry for nerfs (honestly, look at the threads demanding the second boss in Rabanastre be nerfed when he's incredibly easy if one just pays attention), or devs release something mildly challenging > "omg too hard pls nerf" (ShinEx). And that comes largely from the more softcore/midcore side.
Look at all the people that asked for Ultimate; none of the hardcore players have complained about Ultimate's difficulty. All the Ultimate complaints have ultimately come from the more casual side, saying it's "a waste of resources".