Admin warnings do already 'expire', we just take into the account if you have been warned before for any misdemeanor you have committed or not. Each time you purchase the admin rank again, you are eligible to receive another 2 warnings before the privilege is revoked from your account.
There's also the case that exceptions can be made whenever an applicant is submitting their application for the 'supervisor' role on this community. However, the reason for this kind of strict requirement is to ensure that anyone who applies for the position that has been an admin before is disciplined enough to not have received any verbal warnings for their actions before.
There is an excellent way of making that warning of yours 'expire' by time -- do not receive any warnings, participate in the community-related discussions, be active, follow and enforce the server rules, and be a 'role-model' for other admins or possible candidates for the position.
