Isn't this class trinity a bit dated now?

[DarthVella]

The reason we have a trinity system is because it's a tried-and-true formula that's easy to design and balance around... It's much easier, much less of a headache, to design more straightforward jobs with interesting gimmicks than to develop true hybrid jobs.

Agreed. As interesting as a more varied set of roles would be, it would take an intense amount of trial and error for it to be even near balanced, and you would need to fine-tune the system each and every time you add levels and classes. And while doing so is not a bad thing to do, it is (sadly) far easier and more alluring to take something pre-existing that you know works and put your own spin on it.

To cap it all off, to get a new system like that working within an existing game would require uprooting and reworking pretty much everything from character stats to solo quests to dungeon mechanics. You might as well be making FF XIV: Reborn Harder.

[Voldemort]

I foresee a problem with necessity vs. optional-party-member when it comes to preformed parties completing duties.

With the current trinity, (in theory) any job can be brought to fulfill each of the roles. Bringing along a SMN and a WAR should not be any different than bringing a MNK and a PLD. The fact that you bring one DPS and one tank is sufficient for those two slots.

However, if we introduce support roles, there will inevitably become a struggle between needing a support role and optionally bringing a support role. Duties would either have to be balanced to allow both support and DPS, which would be nigh impossible. As clearly an extra DPS would bring in more damage output to kill things faster.
Or the duties would strictly cater to support. Meaning, you must queue with support. And nobody wants that.