A few questions came to mind when I read about the replacement option:
1) Do I have to purchase this now? In other words, am I basically buying an insurance policy or can I simply buy a new battery after 8 years.
2) Does this allow me to upgrade my battery? If I buy a 60 now, can I buy an 85 later for only 12k?
1) It's insurance. You can also self-insure and buy a battery later.
2) Upgrading is an unknown right now. The battery cost latter will almost certainly be more than $12k.