Honestly I think SE are stuck between a rock and a hard place that is half of their own making, and half just the progression of a game. What I mean by that is, as far as I see it: In the beginning, they made the game based around the trinity: Tank, Healer, DPS. Tanks had kits more focused on defense over offense (Think like Flash and Shield Bash, old Inner Beast), Healers were focused on healing, having mostly GCD heals and not as much oGCDs, and DPS were DPS(minus Titan Egi best tank). Healers had dps abilities, but they weren't really a focus, just something to do when everyone was full HP, and to have something in the solo content. Tanks had dps mostly for enmity's sake, having enmity modifiers to ensure they kept threat through the modifier instead of just overall DPS.
Then HW came, and you saw two things. First, and probably the most important, the raid tier that was released pushed everyone into a DPS mentality. See, in ARR dps mattered, but not nearly as much. As long as no one died, and you did the mechanics, you cleared. In Gordias though, you had to do more than just the mechanics, you had to do a lot more dps than what was usually required. This lead to Tanks tanking with their tank stance off for more damage, and healers squeezing in Cleric for more damage, when I feel SE never intended for that to happen. I don't honestly think they ever intended for the main focus of the trinity to change, but it did as a by product of Gordias, and the community pushing it out as a whole to "EVERYBODY IS A DPS". This lead to conflict specifically with the tanks and healers. You had healers pissed off at bad tanks tanking with tank stance off and just eating dirt, and you had tanks pissed off at healers for letting them die because they were DPSing with cleric on or just not keeping them up with tank stance off. Then you had people who weren't comfortable with MTing with stance off or Cleric stance swapping feeling guilty for not being able to do so and hating the general dps mentality. The next raid tier did not have as huge of a spike in DPS difficulty, but the damage was done and the mentality was in place. The other item was that a lot of jobs were considered by the majority too difficult to perform even at a mediocre level(like DRK) and while jobs were probably the most unique at this point, it also pushed out jobs to the point where they were excluded from the PF. You also had a lot of jobs with a CHOICE of offense vs defense, but the issue was because everything was so DPS focused, almost nobody went for the defensive option as it was less damage, like sacrificing a Fell Cleave for an Inner Release, or using Dark Arts for Dark Mind instead of any offensive ability.
In SB, Cleric Stnace was removed. Imo it was removed before tank stance because you will have an off-tank in most fights. There is no guarantee who that OT would be, and it made sense to have them be a psuedo DPS more than the healers because the healers still had to keep everyone alive regardless of who was tanking. Unfortunately you still hat MT's with Tank Stance off so it didn't really fix the issue. The issue mostly being bad tanks tanking with stance off and pissing off their healers or losing threat on a boss. However, the "DPS means everything mentality" was still the name of the game. You also saw the first steps of homogenization between the jobs because that's the easier way to avoid jobs being excluded. Jobs that were deemed to have too high of a skill floor were simplified, but you started to see the first inklings of a power creep. When a new expansion hits, you buff the job primarily based off it's role. Tanks get tankier, healers get healier, DPS get dpsier. This is standard to make the jobs feel more powerful each expansion. The problem is with tanks and healers specifically, is that when tanks get more defensives, healers get more healing, healers have LESS to do. However MT's still tanking on off-tank stance didn't make it feel as bad as it does today. Healers DPS kits weren't as simplified as today, but I feel that was because SE was still not acknowledging that healers HAD to dps. Tanks with STR accessories existed here too, and I remember my healers complaining about bad tanks who had them in DF as well.
Now we are in ShB, and it's at this point where I personally believe SE finally recognized that the community as a whole wanted everyone to DPS. To avoid stance-less MT's they just remove Tanks stance entirely, and to not feel they were overwhelming their healers, healer DPS kits were severely simplified. This is the biggest point of contention with healers, as most healers were absolutely fine with their larger DPS kits and maintaining healing, but presumably SE was concerned that if they were going to accept that everyone was a DPS, having a larger DPS kit on a healer would shy them away from the role. You can believe it or not, but there are still forum threads to this day saying that healers don't need to DPS. Tales from Duty Finder type stuff. Now do I think they took away too much from the healer DPS kit? Probably, but they played it safe. You saw more homogenization amongst the roles to avoid exclusivity, but in the background, you had the power creep again. Tanks got tankier, healers got healier, DPS got dpseir. Again, this was mostly felt by the healers, needing them to not heal or GCD heal as much, but now with a simplified DPS kit, the boredom was felt even more so.
That leads us to where we are today in EW, where I personally think a couple of things happened. First, the power creep. Tanks and healers are the strongest they have ever been. This means that unless you have massive amounts of damage going out, the tanks and ESPECIALLY the healers are going to feel they have bloated, boring kits. The other issue is that SE knows their game has gotten massive over ShB. You have more people playing your game than ever before. So, they played it safe. I think everyone can agree that the dungeons and trials were the easiest ones to ever come out in an expansion(there are no wall to wall pulls in any dungeon this expansion). This further highlighted the simplified dps kits healers have with most of their healing kits being oGCD, and having so many healing abilities in general.
Power creep of defensives for tanks and healing abilities for healers + SE playing it safe in content on their biggest expansion to date by not slamming damage into content even though the tank and healer kits are more than enough to handle it + SE accepting healer DPS but not wanting to shy away new healers who aren't comfortable with dpsing = where we are today. I personally think they best thing the can do is remove/combine healing abilities on healers so that they have less options to respond to damage with, possibly making you have to GCD heal at least once, and replace those open hot bar slots with DPS options. I don't feel they can just add on DPS abilities without taking something away, and if healing is too easy, taking away or combining an ability or two might not be a bad first step. They could also just flat out remove the 30% mit tank abilities and Rampart as well. That would be a quick bandaid to make things more engaging for the healers imo, though that won't help the most top end raiders in scripted content. Perhaps adding more random mechanics or random damage, any healers missing Bosses Crit Auto's yet? Anyway, that's my tin foil hat on how we got to where we are today, and I am sure I mis-remembered a few things to note. I don't think SE fully realizes this, and I don't expect any major reworks to healers come 6.1 as the attitude from Yoshi is "it'll get harder", but we'll have to see.