No argument here, better gear = less to heal. However, any attempt to "fix" this to make healing more engaging effectively amounts to "Defence and HP stats are useless". It's fine for better gear to make content easier over time, it gives that feeling of power and progression as people get through the content. Healers having access to a greater DPS kit just means that they don't get bored later in a tier from lack of healing, simply because they have another engaging thing to focus on.
Also don't disagree, I'd happily be in favour of reducing tank self-sustain greatly and increasing healing checks.
Damage exceeding maximum HP is a mitigation check, not a heal check. Remove the tanks in such a situation and people die, remove the healers in such a situation and people still live, albeit at low health. A heal check is continuous, back-to-back damage such that even with mitigation, you will die. Apart from that, I also don't disagree with the rest of the statement.
This however, I fully disagree with, someone tunnel visioning is not a reason to kneecap a jobs kit. Imagine if we decided that Warrior shouldn't have Fell Cleave, Inner Release, and Infuriate because it "detracts from their tanking responsibilities". Or if we removed the cartridge abilities from GNB (and Continuation with it) because it also detracted from their tanking responsibilities. Or if we removed Healing Waltz from DNC because it detracted from their DPS responsibilities. All this does is simplify a job down to its role and not what that job specifically offers.
Healers can and should have increased healing responsibility compared to now, but that doesn't mean they can't also have more than Glaroilificosis spam + DoT. We know it can work because that's how it worked pre-ShB. SCH had like 7 different attacks back in SB while raiding (Broil, Bio, Miasma, Miasma II, Ruin II, Shadowflare, Energy Drain) and it didn't detract from its ability to heal. Now, it only has Broil, Ruin II, Biolysis, and Energy Drain in that same raid environment, with Broil being spammed much more compared to how it used to and Energy Drain being nerfed since then.