You never know. Just because he's done well for the role doesn't mean they don't want someone else or someone cheaper. Hell, David Hayter was THE voice of Solid Snake for the vast majority of the Metal Gear Solid series, but Kojima made him AUDITION for MGS 3 (allegedly they were trying to get Kurt Russel) and 4, and eventually kicked him out for 5 in favor of Keifer Sutherland.
Also, note that he said ARKHAM games. He never said anything about not being involved in any OTHER games. I'm sure he's done with Arkham but that doesn't mean they're not working on something else.