revealing Chekov as the traitor would have logically written him out of the potential next film.
Would it? After one bold step, another one could have followed. In ST6:TUC, Kirk and friends essentially made themselves redundant: they are the embodiment of the old school that has to retire before the new age of peace can begin. Their fate might not have been all that different from that of the exposed and convicted Chekov. Ashes of Eden
was about post-Starfleet heroes anyway, and a story like that could easily have accommodated a traitorous Chekov in a redeeming role.