You would have to look at the Prius for real world battery life examples. With them, the batteries don't usually just go tits up, they just get less and less efficient. Typical is 150 Kmiles before they need changes. But they don't actually work at the capacity of the Volts. So it might not be a fair comparison.
There is actually a growing business in the Prius world. They are picking up high mile cars with dead/weak batteries for near nothing, Fixing only the faulty cells, and selling them for a handsome profit.
GM covers the batteries for 100,000/5 years as did Toyota. They have gone way over the line to keep the batteries in good condition and also to segregate and monitor the cells so if one goes bad. Cells can be diagnosed with a Tech II and replaced as needed.
Now what that would cost, worst case, I have no idea.
I am at 22 Kmiles now and expect to be close to the 100 K mark at 5 years if not before. I'll cross the battery bridge when I get there.
I have no idea exactly how the battery systems on either car are setup, but wouldnt replacing single cells in the array just drain the new batteries down to the older one's levels much quicker....similar to how we should replace both batteries at the same time on our trucks or risk the new one failing early?