My mother and I picked up a used 2016 Ford Fusion last May, and the battery decided to peter out last month, requiring a battery replacement. It's also worth noting that it's a former rental with a good 40,000+ miles on it when we got it.
Assuming the battery actually came with the car when it was first built, if you leave out any electrical issues, battery aging prior to installation, battery abuse (e.g. lots of deep discharges, which kills lead-acid batteries), and finally, outright defects for a moment, what could cause the battery to die prematurely? In my experience, I've seen batteries last five years or even longer before they go bad.
Also, when it comes to getting used cars, what would you recommend replacing at the outset on acquisition of a used car? In the case of this Fusion, I had the oil changed by the garage that took care of our previous car (I've done the subsequent two oil changes myself; your videos gave me some tips I hadn't thought of, notably the gasket stick issue which I have not experienced yet; my departed dad told me about lubing the oil filter gasket as he did a lot of auto work himself), plus I replaced the cabin and engine air filters. Part of me wishes I'd replaced the battery as well so I am at a "known state", especially given your video about batteries.
Thanks for any ideas!