In counter of your "thin logic" label, I will respectfully disagree. Stealing something makes you a thief. Pulling out a sword and killing someone more than likely makes you a murderer or part of the kings militia. Warrior is different because they are skillfully trained in combat. In medieval times there were the farmers, and then their were knights and there were mercenaries. Your comparison illustrates the farmers in this case. In my system warriors also have special abilities, a form that is widely unique to them.
I think we can agree that the calibre of warrior as a profession differs highly from just a farmer, a paid mercenary and a fully fledged, lifetime trained Knight. Where I will agree there is easily a multitude of thief...roles, as a profession, it still boils down to a set of skills, skills that are relatively common place. Arguably the thief backstabb could be considered a unique ability, this two has been added to the list of normal combat attributions. In this case, a simple double dmg for surprise situations
In fact it is to me far more arguable if anyone would have a better backstab, it would instead be an assassin type Prof, like ninja. The rest of the thief's skills boil down to fairly mundane abilities, henceforth the question. As mentioned thief roles boil down to roleplaying. You can be a good thief or an evil one, a romantic one, one that specializes in break ins, while others might rob caravans, again, none of these roles cry for a specific new profession.
My system as a whole tried to take all the architypes and boil them down to their core, and then were rebuilt with skills and abilities that would help define the unique aspects of the profession itself. Players can take more skills and abilities to accentuate their idea even further. Thief, when you look at it essentially boiled down to a select group of mundane skills. No special prof was required to express nearly iteration of them.
Lastly if that isn't good enough the Mystic/Rogue/Monk Proff covers nearly all of the missing gap the theif would have, plus has extra abilities to boot, which sits rather nicely with my jack of all trades, master of few idea of bandits and would be thieves.