Just an idle thought:

Jobs are currently superior to classes because what they bring to the table (Stat Boosts, additional abilities) outweighs what they remove (access to other classes' abilities).

With the introduction of Rogue (and eventually other classes) there will, presumably, be new buffs introduced.

How many new buffs would it take to make, for example, an Archer do more damage than a Bard? They would lose the Dex bonus from Bard and Rain of Death iirc. Other than that, Bard's abilities are support-based. Bard has access to quite a huge amount of attack buffs already, so at the moment, going as Archer has no real benefit.

Thaumaturge may be in a similar position with Black Mage. They would lose Flare, which is a significant damage spell and convert which also allows more DPS, which is why it's currently better DPS than Thaumaturge even with access to a wider array of buffs.

If the DPS of classes in certain cases approaches that of jobs, would the added flexibility push them over the edge? A Thaumaturge can Swiftcast Raise, for example. Tanks could add DPS abilities as well as new class abilities from the new tanking class to push their DPS on burn phases of fights.

Now, the maths may not add up even with the three new classes we expect to get by next expansion, but there will be expansions after, with new classes and new buffs. Would access to 15 classes' abilities push jobs onto the back burner?

Obviously, there's no way of knowing for sure until we see what new classes bring, but it's something to consider.