If you just close the bypass switch, a large current will flow out of the (dying) cell, making a lot of heat.

You could have a two-way bypass, disconnecting the original cell, but that would cost more. Remember the bypass switch is duplicated for every series cell group (hundreds) and must carry the whole battery current.

Or you could have some kind of slow drain resistor - but then you're back to the time issue.

Okay, I understand. I was definitely imagining some sort of latching switch, that would connect to the bypass instead of the cell. Makes sense that would be more expensive.