Presumably there must be a spread between the average money in from other airlines buying the miles, and the average out from buying redemptions. Although interest rates are not that high at the moment, as it will take even someone crediting 48K a year to Aegean a number of year to build up enough miles to take the whole family on a big award trip, Aegean are essentially getting an interest free loan (perhaps even interest positive) for a couple of years every time a miles gets credited.
Also, I would suspect the number of Gold members who *never* fly Aegean is actually on the low side, even if there are a number who fly them only every 2-3 years. For most non-Greek Europeans it is hardly a chore for you to take a trip to Greece/the Islands for your summer holiday every 2/3years and being an Aegean Gold makes you far more likely to fly on them. Even if you are an America based flyer who usually flies domestic on UA, if you are the sort of person who takes out a FF membership of a European Program, and also therefore sees the potential of using miles to travel outside of the US, you are likely to travel internationally too! And if you are coming on holiday to Europe every now, you are now far more likely to fly on Aegean.
Perhaps they still make a loss overall on it, but I suspect it is not quite as slam dunk as it may appear.