The problem with the idea of "support as replacement for DPS" exists both on the back end and the front end. On the back end, you're dealing with the programmatic complexity of diluting the effects based upon the number of targets and even the roles present (imagine the annoyance a "support" would have if a tank was standing in the buff patch that increased damage but diluted it across all people standing in it; you'd be cutting the real DPS contribution by 25% and that's outside of the control of the support player). On the front end, there are all kinds of issues. You're dealing with the problem of support not being able to solo effectively (notice that healers get Cleric Stance because healing and DPS have completely separate stats; on top of that, healer DPS in Cleric Stance is low enough that they often have to resort to healing themselves while solo) unless you make the support a purely group oriented role, wherein you have the problem of group play being *completely* different than solo play. You're dealing with the problem of your performance being completely or largely contingent upon the performance and behavior of other people, plus you can't really have the buffs/debuffs operate upon percentage gains and instead must make them operate upon additive/subtractive gains to make sure gear progression has some value (i.e. debuffs reduce values by a flat number like 50 rather than 10% and that flat number is determined by your mainstat/weapon).
Support as a replacement for DPS rather than a completely discrete role that the game is built around has *all* kinds of problems, which is why ARR has turned "support" into a secondary role that a job adopts as a side benefit to its existing functionality. You can only really have support as an explicit role if the game is designed around it (i.e. group comp is 1 tank, 1 healer, 2 DPS, 1 support), and, even then, you get a *lot* of balance problems unless you drastically limit the cumulative benefits of multiple support characters being present lest you run into the problems that City of Heroes/Villains experienced wherein the optimal composition was 8 support characters because force multipliers acting upon force multipliers ends up with exponentially improved performance.
The trinity works because you have well defined roles that create predictable compositions that content can be balanced around while preserving both solo and group play in a balanced manner. Support as an explicit a focused role (that isn't just "healing") works in solo games where force multiplication turning a fight into a complete and utter joke isn't really going to create issues (hell, that's a lot of the fun of games like FFT where you can force multiply yourself into unkillability in about 100 different ways). It works in sandbox multiplayers where balance isn't a major concern because that's not really the point of the game. In a combat focused multiplayer MMO, support as primary functionality for a role doesn't work because force multiplication creates a crapton of problems that you actually have to address because you *can't* ignore balance, especially when you want to add it after the fact.
"Support as real role" for a game like ARR is something that has to be built into the game from the start with explicit controls implemented to limit the potency of that force multiplication while not making it a mindnumbingly boring chore that *someone* has to submit to doing for the good of the group. Even so, loads of games have tried to go that route, and I've yet to see a case where it hasn't bitten the developers in the ass. It's one of those things that's *theoretically* possible but presents substantial enough problems to implement practically that it's effectively impossible without more work than could be done cost effectively.
Honestly, I don't expect "support" to become a true role. If they do DNC, I see it as either a DPS/support class, like a melee version of BRD, or a straight up healer. I expect that, rather than trying to create a support role and redo the entire balance structure, they'll go the route of just tweaking the vision of the class to fulfill a role within the trinity: it's simpler, just as effective for bringing the class into game, and doesn't require a reworking of so many other things.