Your example assumes zero down payment, so yes.But if your down payment had been 4k, gap would not have paid out. It's only what you owe, not what you paid.
There were earlier "electric guitars" but I'm thinking all inventions build on previous creations. I don't think you'll find many pure answers to OPs question. I think the closest you'll find is going to be an advancement that produced a single step change in design that flattened the innovation curve forever after. I think the microwave oven was a great example.
Electric fuses also come to mind. Little has changed since 1890.
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