NINJA and ROGUE confirmed

[Exstal]

If Thief does come, it won't be from Rogue. It's as simple as that, really. You need a fundamentally different gameplay style so that it doesn't play the same as Ninja. It's really irrelevant how much one wants to say Thief (as a job) would play different from Ninja if it came from Rogue. Given how, for example, Gladiator / Paladin is 80% GLA/20% PLD and even that 20% is only for a few skills. The challenge is making that 20% so drastically different for two jobs.

Ruling out Thief is effectively the same as ruling out Red Mage, or Scholar, or any of the Jobs people could say are "too similar" or "don't bring enough".

This is simply not true. Look. Thief from Rogue would just be a DPS class. What is Ninja? A DPS class that is heavily invested in Rogue. So how would you make it different? Answer: You can't with the limited job skills we have now (50 cap).

Why add new Tanks or Healers? They'll fundamentally be the same, just with a different name, look and flavour to their skills.

For a different playstyle. That's the only reason you had new classes/jobs. Based on this logic, there is no reason to have Paladin and Warrior. You only need 1 person to take hits.

Why add any new DPS? What is Samurai going to do that Lancer doesn't already? Wield a different weapon? Just give Lancer a Yari type weapon and save the development time, or will Gil Toss suddenly work in XIV?

Again, play style difference. Samurai won't use jump, probably have a move set revolving around their iconic weapon type, probably. Then, it might even be our pseudo-evasion tank. This comparison is poor.

Why add Blue Mage? It'll just be doing magic damage like Black Mage. Blue Mage actually has more issues than Thief would, since the spell learning mechanic would require much more development time. Just slap some Blue Mage gear for Thaumaturge and tell everyone to pretend Fire is a generic Blue Mage skill.

You're right. Why add Blue Mage? There's no reason to add it aside from some people wanting it. Cool. There are classes that I want but unlikely to happen, just gotta deal with it.

No need to add any new Jobs for that matter, just tell everyone to "pretend" that the existing Jobs are the ones they want. Want Geomancer? Pretend Conjurer is Geomancer. Want Ranger? Pretend Archer is Ranger. Want Beastmaster? Pretend Arcanist charms Squirrels. If these examples sound absurd, then good. They are. Just like telling people to pretend Ninja is Thief is.

I think you get the reoccurring theme here; different play style.
-Want Geomancer? Wait for the development team to make a class that uses Geomancy.
-Want Ranger? Define it first because Archer currently has their main skills
-Want Beastmaster? See if development team even cares to put it in

No one is saying Ninja is Thief. No one even thinks like that. The argument is really, really simple. Thief, if implemented, can only come a long time down the road, when there are sufficient job skills to sufficiently differentiate jobs from each other so that the class isn't the driving force. However, if you choose to ignore this part then you have no place arguing in favour of two jobs stemming from same base class because you're being willfully ignorant of the FFXIV class system.

SCH manages to be quite different from SMN with only 5 skills.

ACN (DPS) => SMN (DPS) / SCH (Heal)

Different roles have different focuses. Compare SCH DPS to SMN DPS. What differences do they have? Fester and a DPS pet. What similarities? Everything else.