I'm also still debating lease vs purchase, but as a member of the "forgotten 36" I've got a year to mull it over. I also have never leased, and usually purchase cash and keep the car until it dies. Leasing seemed like throwing money away. But this is different - the value of the Leaf in three years could be unusually low because the battery pack capacity is way down, or could be unusually high because the pack turned out to be robust and gas is $5.00 a gallon. So I view leasing as an insurance cost to allow me the choice to keep the car or walk away in three years. The question is how much am I willing to pay for that insurance, which is pretty much a personal question that's based on my perception of the risk. I still haven't figured that out.
As far as technology is concerned, the only change that would lead me to want to replace the Leaf in three years is a significant upgrade in battery capacity. IMHO, I don't see that in less than 5 years. For a new battery technology to be available in a production car in three years, it would need to be pretty far along in development right now. There's been lot of possibilities talked about on this forum in addition to the article in the OP, but it all seems to be in the basic research stage.