TURIN
What about the main battery failing?
AFAIK there is no MEL item for dispatch with a main battery inop.
If the engines and APU are dead and you are fortunate enough to make it to a runway, the Ship's Battery job is to power the brakes. And if you, and your Ship's Battery is dead, then you're probably going to have a runway overrun. But at least you'll have emergency equipment present and CFRP is strong.
WHBM
I understand that you are from Seattle and keen to bat for the home team, but even there it is surely understood that the recent issues with the battery were not due to it being inoperative - the thing caught FIRE !!!
And I understand that being from the UK, you must love all things Airbus since they design and build the wings for their commercial airplane families there.
Yes, the cabling caught fire on JA829J. And in response, Boeing has switched to cabling that is far more resistant to fire and is capable of withstanding significantly higher temperatures (many hundreds of degrees Centigrade).
They've also encased the battery in a fireproof titanium container and shown that there is insufficient oxygen present within to sustain combustion for a few thousandths of a second even with thermal runaway and electrolyte leakage.