Batteries aren't unrepairable; you wouldn't open one up in the middle of the road to try fix it but at the bus depot with enough volume of battery electric vehicles, they'll have reason to hire repair technicians that can refurbish and repair batteries.
Obviously anything has a bunch of single points of failure, or catastrophic means of failure, but a battery isn't like "one engine". It's basically hundreds of little power modules wired in parallel, so an individual battery cell loss shouldn't bring down the whole pack.
So a battery pack should actually be heavily redundant ... assuming the pack has enough modules for a loss of vehicle to get to some charging station.
