To try and clarify, Thief's Knife is both Useless and forced in our hand.
Mkris, Joyeuse, and Vulcans are not. These are different examples, as i am not a cor, I can't speak if these weapons are replaced by now, However, None of these weapons are
forced in your hand by your linkshell in order to play your job. There is a large difference between those weapons being outdated, and a Weapon thats meant for 70 still forced in our hands at 90 to 99.
We want Thieves' knife because we're still forced to wear it. there is no upgrade to it, there is nothing better to equip for Treasure hunter. Even if there was, it would be a case of "Main hand new Weapon off hand THF knife". Its an outdated weapon we are indeed forced to wear if we want to be on THF.
Again, not a problem on 1 NM fights, a Big problem on Mass farming/Dynamis/Limbus style events