There have been so many Terminator wannabes. Arnold defined that role. Just like only Bruce Willis can be John McClane, only Sylvester Stallone can be John Rambo, and only Harrison Ford can be Indiana Jones and Han Solo, only Arnold can be The Terminator. Other actors can play different Terminators, like Robert Patrick played the T-1000. But the T-101, that's only Arnold.
And yeah, Chris Pine ain't no Captain Kirk either. He's a different character with the same name. Shatner defined that role, just like the guys listed above defined their roles.