I am convinced that time is the biggest single factor in the decline of Leaf batteries assuming that the car is not in an extremely hot climate and the driver makes some effort to not abuse the battery. I noticed that my 2017 Leaf batter degraded at a pretty regular rate even in cold weather and even when it was not driven for a few weeks. According to Leaf Spy Pro it was down about 17.5% in just two months short of 5 years (lost one bar). To me, that implies that the battery chemistry in the Leafs is behind some of the other cars. I don't see the major problem being the lack of active battery cooling unless the car is driven in a very hot climate or if it is driven hard with multiple quick charges per day regularly.
Traded in for a new S+ that has seems to have pretty constant decline in the 8 weeks I have had it, about 0.04% a week. I realize that is really too short a time to really draw any conclusions about the longer term. Since I mainly drive locally, even if the battery declines 30% to 40% over the next 10 years, it will still meet our needs. That is one of the main reasons that we went with the + instead to the 40 kWh battery. It will still be nice to be able to take on somewhat longer trips for at least a few years.