Thieves and Fighters are distinct enough to warrant separate classes. Take away the skills and Backstab (the class's defining features) and you still have differences in HD, attack tables, allowable weapons/armor, and the use of magic.
There's a reason why Thieves/Rogues are included in just about every fantasy game (table or video) ever made and why they are all modeled after the D&D concept of: Fighter strong, slow, focused, and tanky; Thief fast, deadly, skillful, and fragile.
That is just surface level. Every edition of the game has it own nuances of why Thieves deserve to be a separate class. For instance, AD&D and demi-human muli-classing. Feel free to be specific and we can dig deeper if needed.