The T&C seem to indicate that you keep the status for good as long as your account doesn't go inactive for 36 (!!!!) months. Ergo, the expiration dates on the cards seem to be more akin to credit card expiration dates, just to make sure people aren't passing off ages-old cards.
Under B. Gold Card:
http://en.aegeanair.com/milesandbonu...ms-conditions/
5. Member Cards and their Passwords will be cancelled within 36 months if no flight activity is recorded in their Accounts.
This would explain
Skrabban's experience.
It appears that A3's approach, both with the free silver for their credit card holders and the perma-status, is to set a fairly low bar for status—basically giving it to anyone who's ever flown frequently—and cash in on the lifetime loyalty by requiring all the member's *A flight activity to be credited to M&B.