Okay, I think I understand what you're trying to say now. I don't think you did a good job of explaining this if true, but you're trying to say that non-healers that can heal would allow the healers to spend slightly less GCDs on healing and thus squeeze out more DPS? Assuming that would be a concern in the first place, it wouldn't be specifically a tank thing, so I'm not sure why you were trying to make it about tanks. There are DPS jobs with healing resources as well. Why would that be an issue for tank design space and not physical ranged? Or caster?
But there are a number of reasons why that wouldn't be true even with everything else we currently have staying relatively the same, more than I care to list. But the one I'd say is the most telling would be that the amount of GCDs you'd conserve for DPS is not going to lead to a particularly high amount of increased damage. The damage Scholar and Astrologian gain from raid buffs is likely far more valuable than a few extra personal DPS spells throughout a fight.
But in case you missed the literal 5 years of arguments going around, the people who are talking about healer damage balance here are also the ones saying that healers shouldn't have faceroll 111111 DPS combos. I'm not sure why you were completely disregarding that massive point because it's quite integral to the conversation. Because expanding each healer's DPS arsenal will consequently take potency out of their spammable attack spell to be redirected into new attacks with limited uses. In case you missed the last several pages, I specifically have been talking about how constantly increasing the potency of our basic attack spell is hurting the healer experience and adding way more pressure than is necessary for healers to maintain DPS uptime. If we slashed the potency of those attacks and put that potency into other parts of the kit, that also circumvents your worry because gaining a few extra Glares and Broils would mean far less anyway. Moreover, wouldn't you just balance the outgoing damage of jobs without healing support to be at least a little higher than the jobs that do have it? Something we already see to some degree?
Also, what you're essentially talking about is synergy, and a relatively mild form of it at that. FFXIV has had far more egregious forms of synergy, and yet despite that, not only was every job viable back then, but had decent play rates. If Monks were clearing Savage during Stormblood and getting into PF groups with ease despite the Dragoon + Bard meta being a thing, then no, Dark Knight and Gunbreaker would have absolutely nothing to worry about.