a few years back i flew on BA from ORD to LHR, and there was a platinum customer complaining about not being upgraded from business class to first, ms chicken said very calmly and repeatedly "we do not give away our product, and there are no available first class seats. You purchased a business class seat, and we are obligated to provide you with such"
Same situation. Other pax had purchased fc tickets, the op had purchased only a y class ticket, CO was required to supply only a Y class seat, which they did. IMHO, the OP paid for Y got Y. If he wanted first should've paid for first. Simple as that.