Except he isn't the only assassin class in the core books. He's just the only one that happens to be called, specifically, "assassin."
The 1E Assassin can be pretty faithfully modeled with a Rogue and appropriate feat and skill choices - for most characters, 3d6 damage is enough to see them bleeding to death. The more generic killer-for-hire role can be well-filled by just about every class - again, with appropriate feat and skill choices (and, for some classes, spell choices).
This is, I think, one of the main shifts of 3E: you don't need a class, subclass, or kit for every possible character permutation or in-game role. What you need are base classes that are broad enough such that they can be customized to fit any (or most) roles. Want a mounted-combat-focused fighter? You don't the Equestrian Fighter kit or the Chevalier Paladin kit - you just need a Paladin or Fighter to take a certain set of feats.
"Killer for hire" isn't a role that needs a particular class devoted to it. "Secret Cabal of Amoral Killers for Hire, Steeped in Dark Arts, Who Only Accept the Worthy Into Their Ranks," however, has a better argument.