I'm not sure the availability of mileage upgrades has that much to do with how many seats are available. When I used to upgrade on EVA, the customer service rep once told me they had about 2 Business Class seats per flight designated as eligible for mileage upgrade. Once those are taken, no more are issued regardless if the plane flies out with half the seats empty. I know the flight I was trying to update on CX was for late January. I took the same flight/date this year and the whole plane was half empty both ways.