Having an identical, unplugged, fully charged battery with a transfer switch, doesn't really add much complexity.

That depends on where the batteries are located. In this case they are on the other side of the pressure hull. You then get to decide if you want your transfer switch to sit on this side or the other and how you get to pass the signal to switch over from the one to the other (maybe you could automate that). In a design such as this one nothing is really simple and even a silly switch or relay can add quite a bit of complexity. The pressures you are dealing with are immense, it's not as if you're going to run an extra wire with a grommet through the pressure hull on a moments notice. If the cable duct has already been sealed then you're out of luck unless you added spare wiring and even that isn't trivial with this much pressure on the other side of it.

Who said anything about retrofit? This should have been done from the very beginning. I still can't believe they never tested a round trip without people to the depth they were going.

Ah, I interpreted the 'add' as 'after the fact'. My bad, apologies.