Allow a coalition of people, not necessarily from the Board, to revoke our signing key in case everybody or almost everybody from the Board becomes unavailable.
Make it hard for a coalition of people not from the Board to revoke our signing key unless everybody or almost everybody from the Board becomes unavailable.
There is no expiry date on revocation certificates. One way of cancelling the revocation power is to destroy all copies of shares of 2 groups amongst B, C, or D.