Sorry but, you just described the holy trinity in it's purest form. Different people who specialize in different things to clear encounters. Tank's deal damage, but give up damage to mitigate more of it. Healers can deal damage, but usually give up all or most of it to keep their friends alive. DPS forgoes healing and mitigation for pure damage. Damage dealers specializing in different areas of play, would you look at that?
If everyone were treated as a dps role with special things they can choose to specialize in, what happens when you put a group together? You aren't going to take 4 or 8 guys who are all good at mitigating damage, or healing damage, or dealing damage. You're going to find some guys who can take the hits, some guys to keep the others alive, and some guys to beat the living crap out of stuff so you aren't there all day. Pray tell, what is the difference from what we have now?
All 3 roles have an equally important job, and if any of them fail at that job, the group fails. Simple as that. The tank can't hold threat or stop enough damage, he's gone and then the rest of the group has no chance. The healer can't keep up with the damage, everyone goes down. The DPS can't get targets down fast enough, it either kills everyone via the bosses mechanics, or the healer runs out of mp and it all cascades into failure.
If the game needs to be designed with absolute freedom of choice, that means groups who want to specialize in damage, and only damage, will still need to be able to clear any content, since your groups can be formed at random via DF, and a group of nothing but damage dealers becomes a possibility. Would it be fun for you to sit in queue, get into a dungeon, look at each other and say, "Well shit, none of us are equipped to do this, let's drop and re-queue again I guess." No! I want no part of that! Bearing that possibility in mind when designing content in turn means no more tanks and healers and all damage dealers, since it will clear content the fastest and become the obvious choice. Boy that sounds great! It would actually achieve the opposite of what you're looking for.
It also sounds like you're just saying, "Make tanks do more damage, make healers do more damage, so everyone does more damage and the damage dealers are just not useful anymore. So. . . give them debuffs or something, or get rid of them. Whatever. Nothing but tanks and healers!" If you would just look at the skills everyone has you'd realize the damage dealers do provide worthwhile debuffs and buffs to support their allies in more ways than just doing (absolutely necessary and high amounts of) damage. Monks can reduce the enemy intelligence reducing a good portion of damage to the party members, and aoe silence. DRG can buff bards damage. Both can stun. BRD has a broad array of buffs and debuffs. Black mages have. . . err apoctastasis. SMN has Virus, which is again more mitigation for your friends. SMN also brings emergency healing/resurrection. Those abilities while not class defining points, are certainly all relevant (just pretend I didn't say apoctastasis for a sec) and provide group utility. WHM and SCH can each deal decent damage when they get the chance. And tanks deal okay damage. You can't just discount the damage entirely, it is lower but existent. It's not what they're "specialized" in. But everyone is specialized in succeeding. And nobody can do it alone. It's an MMO. It's built upon diversity. And the holy trinity hard coded diversity that works no matter how you put it together (not accounting for skill of course) with varying degrees of success.
Maybe the game is fine and it's your viewpoint that needs a little doing away with in the long run.![]()