The words 'healing covered' referred to the actual 'healing', the act of restoring HP. Presumably, there would be a cohealer in content that demands mitigations, who could have an equally basic core kit, but tuned towards mitigation.
As for BLU's damage, WHM's burst window is that it uses Glare faster, spends a Misery, weaves ONE damaging OGCD (Assize) and reapplies it's DOT. BLU's (as healer) is any of like 5 different primal burst skills, Tingle/Whistle/Triple Trident as a pseudo 123, probably something like Bristle>Matra Magic, and while it does have a 30s DOT, said DOT also has extra fluff because you can Bristle and snapshot it at a higher damage value. Having one filler move like Sonic Boom or Sharp Knife to spam does not make it 'roughly similar'. In fact, you could theoretically even change that to be 1-2 combos for your filler (Peripheral Synthesis > Mustard Bomb, Aqua Breath>High Voltage), or even adapt which filler you're using out of three based on Libra's debuff, should you decide to use Libra
And it's been discussed why a 'stack to 60s' effect would not 'add complexity'. In fact, I previously reasoned that it'd just be 'you refresh twice in raidbuffs, and then play as normal' but now I've been reminded, there's literally zero loss to just refresh it twice at the 1min mark too. Each minute, it's a pseudo 1-1-2 combo of Dia, Dia, Misery. And then you forget the DOT exists till the next minute. It'd be so uninteractive it'd be difficult to justify keeping the DOT in the kit. But we can't remove it from the kit, because we're rapidly running out of kit to even play with, and SE doesn't seem to remove things to replace them with something else to fill the gap, so we'd end up with WHM gameplay being nothing but Glare, and refunding itself via Misery when it has to heal. God that sounds awful to me, but I guess there's some people that would enjoy that. Lucky for them, they can play that right now by just not putting Dia on their bars
Look at it from this POV for a second: I am midcore, I haven't done the latest ultimates, but I have done one (TEA) in the expansion when it was current, so I have at least a bit of experience in on-content-Ultimate (not just overgearing the hell out of UWU). You have done a bit of Savage, mostly the first couple of floors, I assume to get a ring drop from the first so that you don't have to use double tome rings (I also dislike when the Balance recommends ilvl downgrades to dodge Piety).
If you're a self-professed casual, who has not been mid/hardcore, you would find it impossible to truly know what direction the 'mid/hardcore' would want without directly asking them. I would find it difficult to know what casuals would want without directly asking them, but it's not 'impossible', because I was once a casual, back in ARR/HW. I can think back to times when I was not as good at the game, and think 'would that version of me struggle to adapt to X change?' And given that I adapted back then to having one extra DOT (Combust/C2 were seperate) on AST, on top of Cleric Stance, on top of the Cards and Royal Road, I am quite sure that the suggestion of 'Dia duration lowered to 12s, Banish as 15s CD GCD' is not going to overload people's mental thresholds, except for those who have become SO ingrained in the current system, that they would shut their minds to any change whatsoever. In which case, adding 'more healing required' would equally screw them over, because now they can't just netflix their way through the content, they have to react to things
Take Aetherfont's ilvl setup, the ilvl to get in is 605, the gear that drops is 625, the ilvl sync will be 660, probably. But the tome gear we can currently get, is 650! So why not have the 'Sastasha - Hard' style retelling of dungeons return, but actually be harder (ie justify the existence of those CC buttons like Repose), and the 6.4 one drops say, 640 gear? That'd make it equal in ILVL to the new crafted set, but anyone who's serious about Savage would not spam this, but get the crafted instead because A: time and B: pentamelds
Criterion would be a perfectly fine location to add the gear drops to as well if they wanted to avoid clogging the dev pipeline with 'another dungeon to design'. Trade those coins for something beyond just materia and the 100 for the mount