The frequency of failure surely must be an issue. Whilst the regulators might be prepared to accept the better containment solution, are the 787's customers really going to do so, especially long term? How many times are 787s going to land somewhere with smoke coming out of vents or battery fluid dripping down the side, how many ETOPS diverts with hot batteries, and (even if the battery pack is a LRU) how many times will 787s be AOG for lengthy periods because there isn't a spare battery pack at some outstation, before airlines say to Boeing - fix the problem properly?