Buying a new i-MiEV is the challenge, as they are in short supply, and not because they can't produce them.
Factory battery warranty is 8/10 years and 100,000 miles, and Mitsubishi has been good so far in replacing failed packs. In the future, I see independent shops opening up to recondition and/or re-pack batteries with new cells, as prismatic cells can be made from groups of 18650 cells.
No-one knows what Mitsubishi's long-term plans are. So far, the i-MiEV has been an every other year car in the US (2012, 2014, 2016). I'm sure they could really sell the car if they'd run some advertising.
I find that the electric heater itself produces plenty of heat, it's just that the fan doesn't move enough air. According to CaniOn data, the temperature of the heater tops out at 150 °F during preheating with the Defrost setting.