It'll take a lot of convincing for the trans pacific travelling public to believe that there's more chance of 2 RB211s failing than there is of 1 Trent failing. Which ever way you cut it, lose an engine on a 777 and you have lost 50% of your power, lose an engine on a 744 and you still have 75% remaining.
BA can afford to get rid of it's 744s as the 777s that replace them will mainly fly in areas with plenty of en-route alternates. Pacific carriers operating the likes of LAX to NZ and Australia may see it differently.