The Prius also has rebates
The Prius also has rebates but does not suffer from depreciation they way the Leaf does. That is an argument against your idea that it is only incentives that cause the sharp depreciation of the Leaf. It is an issue that battery warranty causes depreciation. The Prius sees this sudden drop when its battery crosses the warranty threshold. It stands to reason that the battery being the most expensive part will see a sharp depreciation when it is close to its warranty limit.