Steal-Stats (STR/DEX/INT/etc)
Steal-Attributes (Evasion/Accuracy/Defense/Attack)
Stealth attacks
Mug (Increases party gil drop by a minor amount of inflicts damage)
Trick Attack (Deals increased damage for next ability and transfers hate to party member)
I could go on, and all of these are just from two FF titles. I'd argue your "NIN as a tank is ludicrous!" line is in fact more ludicrous than "Thieves wouldn't be welcome in a city of sea-thieves in a world that encourages dark arts users, thievery already, assassinations, and will literally in the same patch put in a class that is pretty much always described lore-wise as unethical assassins. There's precedent for one, there is no precedent and (evidence to the contrary) for the other.
That being said, I agree they're not adding Thief. I disagree with the reasoning though. As anyone should, saying thieves wouldn't be morally acceptable with a city of literal sea-thieves being in existence, and now an assassin job is just asinine.