That's understandable, but with a lease you have the ability to find out before buying it. You can always turn in the 2018 Leaf and lease whatever is available in 2021, which is likely going to be the more appealing option given how fast the technology is improving. Or, if Nissan really did hit it out if the park with the 2018 Leaf, we should know for sure by the time 2021 rolls around. I always figured if I knew 100% I was going to buy it, I'd have done so at 0% interest to begin with. I'm leasing because I'm willing to pay a premium to keep my options open.
A good lease payment with a higher but still affordable residual (maybe $16k) is the way to try out a car before buying it. Paying $350 a month with a ~ $12k residual means that if you don't buy the car, you've wasted a huge amount of money on the lease. As for buying with 0% interest, that's fine if you qualify for the full tax credit. Really, though, that would likely be even worse in this case. You'd be buying a car that was only worth
$12k when it was three years old.