Any replacement battery needs to be capable of braking the airplane to a stop
When I was a lad I had a bike with electric lights powered by a dynamo driven by the wheels.
This generator was normally inactive, but at night it could be brought into contact with the tires and it then generated power for the head and tail lights.
Isn't it possible that for emergency use during the landing run of a 787 the wheels could supply adequate electric power for the brakes, even down to very low speeds?
Such a generator might be built into the wheels, or it could be a separate generator mechanically lowered onto contact with the tires in the same manner (kind of) as on my bike. A little bit like lowering a RAT into the airstream.
In short, a battery is not the only way to power brakes in an emergency, and an alternative source might be more reliable (the power source is always there) without being a fire hazard.