They keep up with the heavy HPS check more effectively, allowing them to keep more of their MP to spend on other things. What other things, IDK, I can't claim to understand the mentality of a heal-only healer that well, but I would assume that they'd be happy with the result of 'the party was getting hammered by 25k hits over and over in Harrowing Hell, and I kept them alive through it because I am good at healing'. As said before, 'more healing' can't be the reward because the party is already full HP, the limit of how much effect the healing can have has hit it's maximum and any more is a waste. The only other thing is 'opening up space to deal damage' because the only limit of effectiveness on 'more damage' is 'the boss dies' and that's kinda the goal of the fight
1: Healers not being needed is a symptom of a much larger problem: that offhealing, tank selfsustain, non-healer damage mitigation utility etc has gotten so plentiful, the requirements of the fight do not keep up. And yet, because not every class HAS that utility, the devs cannot adjust the fight design for it. They can design around the assumption of 2 Feints, 1 Addle, 1 Tactroubasamba, sure. But Minne vs Curing Waltz vs Dismantle, we don't know which P-Ranged is going to be present. So they can't make the fight 'expect' a 10% mit tool, because only MCH has that one. Thus, the weird paradox occurs where the tools are given to us, but not really 'needed' per se. Imagine the outcry if players went into a week 1 run of Savage, last floor, and got walled because their comp of NIN/DRG/BLM/DNC couldn't deal with the mit checks, because the fight expected them to be running RDM and MCH for Dismantle and Magic Barrier, the outcry would be massive. Make the utility stuff less mit-related for DPS, they can keep the 10% healing up from Mantra, Minne, Barrier, etc those are cool and interesting, and help boost the power of the shields from the healer. But Addle and Feint are a bit questionable to me, especially considering they, combined, were an ACN skill originally (meaning SCH had native access to them). Addle's effect was limited to only SMN/SCH because of the trait, and Feint was crossclassable to any caster/healer, but now it's throw to melee for some reason??? Guess they need to give the role something to think about, now that they don't have to think about keeping uptime, or positionals (at this rate they're gonna remove them in 7.0)
2: Healing, in a vacuum, is boring to me, yes. If I press Glare over and over, that gets old fast. Changing that to 'press Cure 2 over and over', does not really fix that issue. It's still one button being mashed. It's the dancing between the two sides, of healing and dealing damage, of triage, of quickly reacting to issues that might arise, that make healing 'fun' to me, and this game has a distinct lack of 'thing has gone wrong, quick fix it' moments outside of progression. Why can't bosses occasionally throw out a random autoattack (albeit a lower potency) at a random non-tank player? If the tank takes 80% of the autoattacks, but the other 20% get flung randomly at the other party members, it at least asks the healer player to 'hey heal this guy up, because if the boss chooses to AA him a second time, he's probably gonna die to it'. You'd solve it in an EX roulette-level boss with like, one Solace, Essential Dignity, Lustrate or Druochole, but the point is that it helps to eat away at the resources of the healer. For a comparison, think of two bosses in WOW. One is Patchwerk. He stands and he hits hard, you rotate CDs, and eventually the DPS deal enough damage to kill him. Alternatively, take a fight like Broodkeeper Diurna (dragonflight S1). The boss has to be kited near eggs so DPS can use her own spear to destroy them, and adds keep funneling into the arena which have to be kept 50y away from the boss or they become superpowered. Despite having less 'tank' things to do (there's no tankswaps, for example), it's a much more engaging fight for tanks, because they're constantly having to react to things. Move the boss, move the adds, oh no an egg was missed and now a Nascent add has spawned, ok but the add tank has taunted it, took it out of the 50y range and now it's killable, we salvaged the pull. My least favorite fights in that raid were Terros and Sennarth, because they're 'stand and hit' and the tanks have no agency over the boss's positioning. Dathea is on thin ice, cos she's locked at the middle of the arena, but scrapes by because the TB being 'you get launched further per stack of debuff' is funny to watch at 4 stacks. Conversely, my favorites were Kurog and Broodkeeper, because the tanks have to keep moving the boss around the arena as the fight progresses.
3: It's a complex question, but as it stands, yes to me it kind of is. Pressing one button over and over, as said before, gets old fast. What keeps it interesting is the reacting to things, I don't know the fights 100% yet, etc. But that will fade away over time as I get more practice. The problem is not just 'DPSing is boring' though. It's that, where other roles get rewarded for getting better at their class with being able to do their job better (ie tanks do a better job of mitigating damage = they make the healer's life easier), our reward for getting better at healing, rationing out CDs etc, is that we get to press our one button more often. Additionally, when our 'one button, more' is the reward, it starts to have a more pronounced effect on damage variance, which leads to potential issues with balance re: enrage timers. If someone casts 100 Glares, but couldn't hit enrage, the only thing they can do to improve that is either check their DOT uptime is good, cast more Glares (sometimes not possible due to encounter and mechanics), or just 'crit more lmao'. In fact, a friend of mine showed me a log comparison between their WHM, and the 'current best (at the time) WHM run' of P1 for TOP. Both players had the same Glares, Dias, Assizes and Misery casts. Every 'number of casts' count was the same. But the 'current best' WHM had 100k more damage in that phase, because they crit more. There was zero skill difference at play, my friend's WHM just 'got unlucky' and there's nothing that can be done in that situation except 'go again'
Lastly, 'getting to do DPS isn't rewarding' is one of those things. Like if we flip it, 'being forced to heal a player who keeps standing in orange circles isn't rewarding' is also pretty true, you're not getting anything from it except higher blood pressure. But we do it anyway, because it's part of the job we signed up for when we queued. Additionally, I could say the same for say, Tank. 'Getting to do damage as Tank isn't rewarding', except... It is. When you deal damage on tank, you generate aggro (not that it matters much now), you get selfsustain via your 123 combo, you build resources that lead to stronger hits with additional effects, and you get cool looking CDs to use, that also do damage or have additional effects such as generating other resources. So the problem with 'doing DPS as healer isn't rewarding' might not be because 'its not part of the role's duties', but because the devs have designed it in a non-rewarding way.