I'm pretty sure it's the same with all other Star Alliance members, that have "saver" or "promo" award space and at the same time "Flex" or "full fare" rates, regarding what tickets Aegean can offer for Miles&Bonus redemptions.
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Considering what heights prices for premium cabin products have reached, I'd certainly consider paying those higher but almost always available rates (however, as you said, that's not an option with Miles&Bonus).