Call me crazy, but I think the 2018 Leaf can be a 15 year car, depending on what you mean by "car."
In my area, the 2018 can be picked up for under 20K. This is partly why I think the 2018, criminally crap battery that it is, has actually really good long-term value. Some interesting things to consider about the 2018:
1. It will never have less than 100 miles of range during its 8 year warranty.
2. If you drive economically, and are able to beat EPA ratings by an average 25% year-round, then you will never have less than 125 miles of range during its 8 year warranty.
3. Battery degradation is not linear, but if the battery degrades at an average annual rate of 5%, then it will degrade to 65% after 7 years, which would trigger a warranty replacement, which would conceivably mean getting a new battery that takes 7 more years to reach 65%, which means the car will always have at least 65% capacity (97.5 miles EPA; 121.9 miles "economic" driving) for 14 years.
4. Even at 40% capacity, the 2018 would still have 60 miles of range EPA, 75 miles of range "economic" driving. That's still considered very usable by 2015 Leaf standards. And I could be wrong about this, but I don't see the 2018 battery degrading to 40% in faster than 15 years, simply because the warranty replacement would interrupt the timeline. Even at 8% average annual degradation, warranty replacement would be triggered after a little more than 4 years, then 40% degradation would occur about 7.5 years later, so OK, let's say it takes 12 years total to get to 40%. This could happen. But I don't see the Leaf having 8% degradation consistently over its entire life - I could see it having rapid degradation in the first 3 years and then this degradation leveling off; or I could see the reverse, slow initial degradation followed by steeper end of life degradation. And even if it did have reliably 8% degradation every single year, a $19K car used over 12 years still seems to make a lot more sense than four separate 3 year, $12K leases ($48K total) over that same 12 year time period.
This could be naive, but my 15 year plan for the 2018 Leaf would be this:
For the first three years, use it as a commuter car (my work commute is 22 miles round trip), a weekend car (I drive 50 miles or so on weekends, spread out over 2 days), and a medium-range road trip car (roughly six 250 mile trips to visit family annually). After three years, I figure the battery degradation will be enough to make these 250 mile road trips annoying, so I would then rotate the Leaf into purely commuter/weekend mode, and stay in that mode for another 12 years. Three years from now, my wife's gas car (a CRV) will be a candidate for replacement, and my hope is that we could snag a low-mileage used Bolt, Tesla, Niro/Kona, Leaf E-Plus, etc. for around 15K. And then this used 200+ mile EV, with much more competent battery tech, would be the new long-range car in the family. Anyway, such is the dream.