Healers that don't want to DPS are like DPSes who refuse to move out of AoEs.

[Atos]

...look up the statistics for endgame dungeons (since we're talking casual content)...

First, you said "healer casually DPSing", not "healer DPSing in casual content":

...DPS gain from a healer casually DPSing is more than what 2 DPS could achieve if...

In dungeons where the tank mass pulls mobs so you can AOE them for big numbers is one thing, doing it in savage raids or ex primals between healing and mechanics is another. The latter is not what I would call "casual dps'ing", and the healer's opportunity to dps depends on their class (SCH is more capable of dps'ing between heals), what fight you're doing and how familiar everyone is with the fight.

In same tables you will find groups which did just as well without healers dps'ing much (talking about harder content like savage and primals). In the end, if healer DPS is that much of an issue you really need to take a look at the damage dealers in your group.