I'm sorry, but 7500mi or 18 months is not reasonable. The inconvenience and unpredictability have lessened the value and usability of the car to me. I'm getting a new battery today, but this constant and sizable range deterioration is unacceptable. Even an iPhone battery doesn't fall off like this and look at all the crap they took over it.
I agree that 18 months with only 7,500 miles is not reasonable if your battery is really deteriorated to the point of needing replacement. The original battery in my 2011 was 27 months old with probably about 25,000 miles (don't have records handy) when it was replaced after Nissan called me to start the replacement process. Where are you located?
Edited to add: Although I have no intention of replacing my 2015 for a long time, I would probably buy a new LEAF if something happened to force replacement. Both my dealer and Nissan have treated me well so I would definitely consider another LEAF in spite of all the Nissan bashing on this forum. I consider the rate of battery deterioration I am experiencing with my 2015 to be reasonable for the climate and conditions I drive in. I anticipate reaching 60,000 miles on the original battery next week with 10 capacity bars left so I will probably get to somewhere around 80,000 miles by the time it drops to 8 capacity bars.