First, tell me which part of the game isn't heavily relying on tank/healers ?
I've done all the content up to levi Ex and T5 with both my i95 WAR and i84 bard. I've been up to T2 and garuda Ex on my i75 DRG.
Well, my experience so far is : dps is ez mode. You only have to care about instant kill mechanics, while tanks and healers have to deal with a lot more mechanics in between. And don't tell me "there are dps checks", because these are only here to know if you have enough gear to beat the content, or if you know your job.
On the other hand, healers (from what I see, since I have no healer at end game) have a lot of stress to make up for the group's mistake (if they are forgiving...), and tanks have to be careful about the horrible positioning of their comrades, the AoE hate rampage of some healers, the lack of focus of some teams, etc etc... The whole game revolves around tanks and healers, and dps are just here to dance around doing the missing dps to kill before timers.
What if, for some fights, tanks would just have to tank (aka taking damage for the party) without dancing around to avoid 10s of AoEs, while making it easier for dps in the same time ? What if healers would just have to heal, without having to wonder every damn second if they won't make one of the dps die if they continue to save the tank's ass ?
What if dps had the difficult part of the fight for one time ? Just like, you lose if your dps isn't good enough, not because one of you lagged/failed a jump/was learning ?
(of course if you're learning it would still be difficult in some way, with a few mechanics. Key word is : "few")
The last patch (well it's been the case since Ex primals) introduced only fights with "1 death => wipe" for the masses. "Normal" people can't recover from a death in SCoB, or in titan Ex, or similar fights. Nothing is really difficult as far as I can tell. It's just "DDR : ARR", and you get "Game Over" screen if one of the 7 other players misses a step. This is BS. There is no difficulty in one-shot mechanics. You just die the time you learn them, and then it's a perfect score every single time you attempt it.
And yeah, I said "not difficult", because a fight you can do without any errors just because your learned the pattern is not "difficult".
I played a few obscure korean MMOs, which relied on really hard bosses which required like 50-100 players to clear them, with like 3 or 4 AoEs in total. All the AoEs were instant death for non-tank players though, but the majority of the fight was mostly "survive the attacks, kill the adds popping, pary for the boss to die before enrage". But on the other hand they were on the shitty "grind your level and gear with 1 to 5% probability to loot/enhance your gear" soooo... I don't miss them that much.
That said, many western MMOs are going the "pattern" way, because it seems harder at first (yay, hardcore players will be happy the time they beat it) and then become piece of cake once you master them (yay, mainstream/casuals will beat it in 3 month/2 years, they'll stay). I'd say it's a right "commercial choice" if you want to keep people playing for the fights.
If you really want them to stay though, you gotta give them something to do. Atma was in a way a step in the right direction (socialization, even though it only lasted 3 weeks). The union feature they are planning for 2.4 *CAN* become a really good feature for socialization too. Treasure and monster hunts, as well as exploration log, will add things to do if you want to explore the world with friends. But well, for now that's all. As many said in various topic, you can't go in a dungeon with like 6 or 20 people, just for the fun of it.