The point is that for something to be a fully-fledged product, it has to have certain features. They're expensive, so an MVP skips a lot of them in favor of getting a product up quickly. But you still have to add those features in later. They're usually not optional.
Hence, an MVP that you can't add those features to to is not an MVP - it's a prototype. You will get beaten in the market when someone else adds whatever your product is missing.
- if it will (or should) be thrown away at the end of development, it's a prototype. MVPs are not thrown away. Though from looking at wikipedia, even they confuse the two.