RNG branching from ARC is not a new Idea and has been brough up and agreed on a lot of times. And No, Archer is not Ranger. The same way Paladin is not Gladiator and Warrior is not Marauder. The same way Ninja will not be Rogue and Thief is also not Rogue. Archer don't have to work 100% the same like it did in previous FF Titles and the same goes for THF. Actually Bard has proven, that it can differ a lot from how it plays in comparsion to other titles.
We don't know how Rogue and Ninja play, but from what has been said so far, they play different in most aspects. Being Class and Job and using the same Weapon, does not mean they play the same way. THF can still be DPS and also can be "Support". Yea, you are right.. support is not a role.. yet, still can work as one, the same way BRD can work as one, while being a DPS.
Same thing again. Look at how SMN and SCH are different from each other, even if you claim: (90+% of a class/job's functionality is derived from the class, not the job) So why would that not apply to THF and NIN?
5 you want?
1) Steal
Cast Instant, Recast 480, TP Cost 500
Can steal an Item, which would drop from the Treasure Chest, after defeating it. Chance of Success: 10%
2) Treasure Hunter
Cast Instant, Recast 720, TP Cost 250
Improves the successrate of Steal by 15%, lowers your defense by 15% in exchange.
3) Sneak Attack
Cast Instant, Recast 180, Duration 30 sec
Must be executed from behind, with PT member between you and the Target.
Increases your Atk by 15% if successfull, STR of the PT Member by 5%, lowers your defense by 15% if failed. Successrate: 70%
4) Trick Attack
Cast Instant, Recast 360, Duration 30 sec
Must be executed from flank, with PT member between you and the Target.
lowers your Atk by 15% if successfull, lowers Atk of the PT Member by 5%, increases Steal by 5% and next SA by 30%.
5) Gil Toss
Cast Instant, Recast 480, TP Cost 200
Does throw a random number of Gil out of the PT total pool ( max 50k ) at the Enemy and in exchange can incease the Droprate of rare Items. In example Mounts.
Lol.. i made those up in like what? 2 Minutes? The 5 Job Abilities can change the Class and its gameplayA LOT.
It is the way how you choose the Abilities and how the actual Job can make use of the Class Skills. Let's just assume that there will be THF beside NIN, they both don't have to share the same Cross-Class Abilities and also do not have to share the same Base-Stats. They also could be a different role, without being both DPS or both can be DPS, but one can be supporter and the other "Debuffer". IT is quite possible, so i don't see a Problem here.
I am not saying Thief is coming, or should come.. tbh i do not really care, but to say it is impossible is just not true.