The only potential legitimate justification for former spooks keeping a security clearance is if the current Administration would want to give them access to classified information to seek their input on something. But when you have someone as publicly and bitterly opposed to the Administration as is Brennan, there is zero chance of that happening because the Administration could not trust his advice.
Therefore, his opposition to the Administration destroys the only justification for him keeping his clearance, and it should therefore be revoked.