Lithium battery costs are now down to $115/KWh, and there's good reason to expect them to drop further. But even if they don't, a replacement cost of around $4k for a 40KWh battery seems feasible, especially compared to the replacement cost of an ICE engine
The biggest issue I would think is not one of cost but of compatibility as the tech changes. Early Nissan Leaf batteries for example, didn't have heat sinks in them which is why they degrade comparatively quickly compared to current stuff, and even then they are outlasting original estimates by quite some margin.
Our Mini battery is just over 3 years old. No measurable degradation in range, no sign of any problems. I see no reason to think it will not be still providing useful capacity way beyond its 7 year guarantee period. Even if it degrades to 80% capacity it will still be fine for what we use it for.
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