Quote Originally Posted by Nahara View Post
Thief doesn't have to be "a worse NIN" and Ninja doesn't have to be "a better THF"; they can be thematically and mechanically different.
Underlined: No, they can't. They share the exact same methodology of attack. Fast, low damage, high critical chance striking.

They have only been a few games within the meta series that Thieves are not combined with Ninja's or replaced by them out right.
FF5: Thieves retain viability due to Thief's Cuffs item that only they and Suppin can equip.
FF6: Locke and Shadow have very similar stat growth and share a majority of gear.
FFT (series): Thieves are given an entire suite of Steal schticks that allow them to separate them from Ninja's

For thieves to be their own class, they would have to give over most or all of the respective job skills to various stealing abilities that could lead to thief being non-viable from a dps perspective.