The implication, that one should make a plan for that, is a valid reminder not enough of us probably do.

But it hardly needs to be difficult. If you're running dovecot and postfix on a server somewhere then yes, family is screwed. But it's simple to use either some mail forwarding service that you pay for with a credit card, or something like fastmail (etc). Leave 2 pages of instructions for how to log into and renew the domain (print the QR code used for the 2fa enrollment!) and how to log in and pay for whatever the underlying services are. Place in a binder and label "Family.Name Email Management" and put it with your other important documents.