The specialist teams that were brought in provided fixes for all the potential problems and those that had been found.
The key word here is potential. By Boeing's own admission, they don't understand the root cause yet. 'Yet' being my own addition, as I haven't heard that they will continue to look. The Big Battery Box may end up as a permanent solution.
If Boeing does discover the cause and goes back to the drawing board with an updated design, the FAA (and others) may give them a clean sheet of paper, so to speak, on battery reliability. Eventually, the battery box could go. However, if they continue as is, how much reliability data do they have to collect with the current 'probable fixes' in order to demonstrate something on the order of one event in the life of the fleet. I'm not going to do the numbers here, but my gut feeling is that this will only be provable over a significant fraction of the fleet's lifetime. Hence the perpetual battery box.
I know a few of the engineers left at Boeing. This problem goes well beyond their remaining base of expertise. Some smart battery subcontractor may find the solution and Boeing may put its stamp on it. But recertification is going to be expensive and not likely something a sub is going to finance out of the kindness off their heart.