At the end of the day, the fleet decision will be about CX's needs in 10 or 15 years time, not 2012. The 747 has certainly served CX since 1978 and there's no doubt that the 747-8F will serve it well on the cargo front, BUT the 747-8 is the end of the 747 line; the A388, even though it mightn't be that attractive to CX now, is the start of the 380 line.
Airbus will certainly launch the A389 and they are well aware of the comments of carriers like CX who have been pushing for a stretch. They won't let CX go without a fight and when it comes down to a battle between the 748i and the A389 ...