I don't think you should be able to have a badge expire. You learned it at the time and met the criteria. If the criteria change--that's a different badge & I think you'd need to earn that one.
I'm not sure how I feel about revoking, but believe it might be necessary if the person contravenes what the badge stood for. For example, if someone earned a badge for digital professionalism, then did something completely unprofessional in a digital environment that got back to the issuer, I think the issuer might need to have the power to revoke in order to maintain credibility.
Also, what if a system got hacked and badges were issued that needed to be revoked later?
Julia