The battery wears out depending on the user. No battery can be warrantied forever, and there are many different applications that affect battery life. You know that I-phone warranties the battery in my 7 phone for 500 cycles saying it will have at least 80% capacity by then. I'm already at 786 cycles and I'm down to 86% capacity.
So let's suppose Ford did warranty the battery, and they did for this particular person, mipmapped. They replaced his battery and within 1 to 2 years his new battery was already worn out again...
As for the car, some people can commute to work on a battery charge. Some people get there and can charge there to come back. Imagine those people who charge the battery twice a day to go and come back to work, how long do you think its going to last? Remember that many people are trying to use the small battery to drive the car around as if there is no engine. Some people charge, go to work, charge, go to eat lunch, charge, go back home, and charge, go out to go shopping all the in same day.
Remember that the more you charge and discharge the battery, the hotter it gets and extended times of being hotter affects battery life also. Hot summers don't help either. The point is, if there was a warranty on the battery for wear, I'd bet almost everyone can get the battery replaced within the warranty period by using the first battery exclusively and saving the engine for the 2nd half of the life of the car.
I'm already on my 2nd Cmax, unfortunately mine got hit (#1) and the insurance totaled it and I ended up buying another knowing how well the car performed and drove I couldn't be without it. Sure I understand the limits of the battery and the heat needs to be watched, and I try not to use it for any stressful things if the engine is hot and available on longer trips. Some people never bothered to piecemeal the battery out, they left the house in Auto, drove off and stayed on battery until it was drained and the car automatically switched over to the engine. Sometimes draining the battery in 15 minutes or less by driving 70mph on the highway. None of that helped of course, this forum has been a great asset to all, with loads of information on the to do's and the to don'ts, but some people didn't want to listen or ignored the micro management of the battery hybrid system. Now here we are and it is what it is. Just remember that no matter what, if you want to use the battery exclusively its not going to last as long. You'll end up with a worn battery and a new engine. Maybe best to share the load more even if you do the engine still ended up being used about 1/3 to less than 1/2 of the time as in hybrid mode you still coast without the engine.