I guess we just don't know what it will cost to replace a battery outside of warranty so it is an unknown being in a new market. We can't really predict what the future market will be like. There might be aftermarket support, however, Honda plans to sell 75,000 Honda Clarity PHEVs. This might not be enough to get the aftermarket interested. My hunch is if the battery fails at 8 years the cost to replace the pack might be as much as the car is worth (or more).
The Prius sold many more cars than the Clarity plans to sell so battery replacements for an 8 year old car would certainly be easier to find and better value. Tesla Model 3 will clearly be selling in large enough volumes to get better support while used, but currently Tesla's issue is they won't sell you parts for the car so you could run into issue being able to get OEM parts without going to a Tesla authorized service center.
My advice is buy what you are comfortable buying. If a failed battery totals out the car later, so be it. My guess is the car won't be worth more than $8k in 8 years anyway. My guess is a battery will be less than that, but still maybe $4k. The rest of the car could still be sold for parts and possibly a smaller battery could be retrofitted to make it usable, etc.