Unfortunately with the 787 the chance of battery thermal run away was considerably less than 1 in 10 million
Over a hundred batteries have failed in 50,000 hours, which gives a 1:500 hours failure rate. Over a 10 hour flight thats 1 in 50, the chance of both batteries failing is therefore for 50x50=1:2500. All year flying with a 500 aircraft fleet means 70 dual battery failures per year. And 3500 single battery failures per year.
Mitigating this is Boeing claim that most failures were due to over discharge (which is obviously impossible as its basic preventative circuity requirement). And the less strenuous charging/discharging could make a huge difference - but they would have to find some method to proving this theory with a lot of testing