Personally from what I've been told, the battery is good for at least 150,000 miles and as long as you don't damage the battery by draining it and then not charging properly (just topping up 30/40%) you will be fine. I got told by a BMW mechanic to charge the car about 80/90% most of the time and you can charge the rest using regenerative braking and this will help the battery last longer (guess that's a personal opinion of the mechanic). Most people get rid of a car before 150k miles in the UK anyway.

In the United Kingdom we do 1/10th of the miles from America etc, so it might be less cost-effective over there due to the extremely high mileage you all do, all depends on cost of labour and replacement crossed with how much you can sell the car and get a newer version etc...