@Old Engineer
Cell balancing is nothing new, used with large telco installations for the better part of 100 years.
In small batteries (up to starter battery size) Ni*, Pb* took care of itself well enough for practical use. However, cell imbalance is present there too. In a typical old car battery, one or two cells will typically cause the need for replacement. These cells were often impaired by imbalance.
What's newer is the need for precise management of smaller Li* batteries. Every small battery has circuitry (these days often a single chip) built in.
For small cells up to a few Ah, balancing by resistive means is sufficient. The differences amongs cells are just a few percent, a balancing current in the 10s of mA is sufficient. Important is to prevent cell over/under voltage.
Discharge balancing is never done in high load applications. The available capacity is limited by the weakest cell.
Individual cells, which dropped capacity by a few percent are routinely replaced in large installations. Some, like stationary 48V Pb telco battery cells can be refurbished. I guess that it would be possible to replace individual cells of reduced capacity in the 787 battery.