Why would you need to replace the batteries? Do they fail outright at around 10 years, become unsafe, or do they just lose capacity?

Curious!

Even if they're at 50% capacity, they would still work, right? But if there are other considerations, especially safety ones, then that would definitely be a consideration. I'm not sure where to learn about this type of thing.

> Do they fail outright at around 10 years, become unsafe, or do they just lose capacity?

LiFePO4 generally degrades to 80% capacity after 10 years, that's it. Safety isn't an issue.

At which point if you're short on capacity (but who knows how your demand might shift over a decade) it's not like you need to replace the original batteries to get that 20% back, you will probably be able to just expand the pack to bring the capacity up.